Symbiosis and Communion

The Second Self

Everyone asks what the symbiote gives them. Nobody thinks to ask what it keeps.

— Vessa Thrym, doomed guide

Symbiosis is not a bargain struck once and forgotten. It is a conversation — long, patient, never quite finished — between the person you are and the living thing you have invited beneath your skin. This chapter gives that conversation its rules: how to bond a symbiote, how to spend the quiet resource called Communion, and how to reckon with Infestation, the slow price the Deep exacts from those who lean too hard on borrowed power. All of it is built for the 2024 rules and tuned to sit beside them — optional power carried at a cost, never a free bonus. The character who never spends a point of Communion loses nothing by it. The character who spends freely will, sooner or later, be asked to prove whose hand is really on the blade.

Bonding a Symbiote

A symbiote is a living organism of the Weave Below — a blade grown from chitin, a mantle of luminous cilia, a spore-beast that nests along your spine — that takes up residence in your flesh and your fate alike. Any character may bond one. You do not need a special feature, a class, or the Sporesung's blessing; you need only a living symbiote and the willingness to let it in.

To bond a symbiote, you spend a Short Rest or Long Rest in communion with it: still, quiet, breathing its spores, letting it learn the shape of you. At the end of that rest, the bond takes hold. You can bond only one new symbiote per rest. A bonded symbiote grants an always-on Boon and one or more Gifts you awaken by spending Communion (both defined below). While bonded, a symbiote moves with you, feeds from the Weave you carry, and cannot be taken from you by any effect that targets objects — it is not a thing you wear but a second self you house.

The Three Bonds

You can maintain a bond with at most three symbiotes at once. This limit is separate from — and parallel to — the usual limit of three attuned magic items. A symbiote is a living creature, not an object: it never counts against your magic-item attunement, and attuning to a magic item never uses one of your three bond slots. A character at the height of their power might carry three attuned items and three bonded symbiotes.

That is deliberate. Magic items ask nothing of you but a slot; symbiotes ask for Communion, and Communion spent greedily becomes Infestation. The pressure valve on symbiosis is never scarcity of slots — it is the living cost of using what those slots hold.

Boons and Gifts

Every symbiote grants a Boon: a modest, always-on benefit that costs nothing and asks nothing. A living blade you can never be disarmed of; a mantle that hardens into armor; light that answers your breath. Boons are roughly uncommon in power and are yours simply for carrying the bond.

Every symbiote also grants one or two Gifts: sharper powers you awaken by spending Communion Points. A Gift costs 1 to 3 CP, scaled to its strength — a 1-CP Gift rivals a good Bonus Action option, a 2-CP Gift a low-level spell, and a 3-CP Gift an encounter's turning point, never a nova you can repeat.

Communion

Communion Points (CP) are the spendable measure of how deeply you are willing to reach into the bond — and, quietly, of how much of you is still wholly yours. Your Communion pool is small on purpose. It cannot be spammed, and it is not meant to be.

Your Pool. Your maximum Communion Points equal your Proficiency Bonus plus your Constitution modifier (minimum equal to your Proficiency Bonus plus 1). Constitution is the anchor of symbiosis throughout this chapter: it sets your pool, it steadies you against Infestation, and it powers your symbiotes' Gifts. A character with no bonded symbiotes still has a Communion pool; they simply have nothing to spend it on.

Regaining Communion. You regain all spent Communion Points when you finish a Long Rest, and you regain a number of points equal to your Proficiency Bonus when you finish a Short Rest.

Communion Points by Level

*Your pool equals Proficiency Bonus + Constitution modifier (minimum PB + 1). The values shown assume a Constitution modifier of +1 to +2; a higher Constitution raises the pool point for point.

Spending Communion. You spend Communion Points to activate a symbiote's Gift, paying its listed cost from your pool. You cannot reduce your pool below 0, and so you cannot pay for a Gift you can no longer afford — unless you choose to overreach.

Gift Save DCs and Attack Rolls. Unless a Gift says otherwise, a Gift that forces a saving throw uses your Symbiote save DC, equal to 8 + your Proficiency Bonus + your Constitution modifier. A Gift that makes an attack roll not tied to a weapon uses a bonus of your Proficiency Bonus + your Constitution modifier. A Gift that attacks with a symbiote weapon uses your normal attack roll with that weapon.

Overreach

Sometimes the pool runs dry and the moment will not wait. When you activate a Gift, you may overreach: pay any part of its cost you cannot (or choose not to) cover with Communion Points by drawing instead on the bond itself. The Gift takes effect immediately. Then you pay the true price.

Track an overreach tally, starting at 0 after each Long Rest. When you overreach, add the overreached portion of the Gift's cost to your tally, then make a Communion saving throw — a Constitution saving throw against DC 8 + your overreach tally (minimum 10). On a failure, your Infestation rises by 1 (see below). You may split a Gift's cost between Communion Points and overreach; only the overreached portion is added to your tally.

Because the tally climbs with every reach, overreaching twice in a fight is a gamble and overreaching three times is a dare. This is intended. Overreach is the door the Deep leaves open for the desperate — and it charges by the step.

Infestation

Spend carefully and the symbiote remains a guest. Push too hard, too often, and it stops asking and begins to decide. Infestation is a six-step track measuring how far the balance has tipped from you toward the thing you carry. You begin every character at 0, Your Own, and pray to stay there.

Rising on the Track

Your Infestation increases by 1 whenever you fail a Communion saving throw. You make a Communion saving throw (Constitution save, DC 8 + your overreach tally, minimum 10) whenever one of the following occurs:

Overreach. You pay for a Gift with the bond instead of Communion Points, as described above.

Blight Exposure. You start your turn engulfed in blight-fog or Rustfall, take damage from a Blight Below creature's spore or rot attack, or consume something touched by blight. Make one save per such exposure.

Falling. The first time in an encounter that you are reduced to 0 Hit Points while bonded, the symbiote surges to keep you — and to take you. Make the save as part of dropping.

Greed. The first time in an encounter that spending Communion Points empties your pool to 0, the symbiote tastes how much you needed it. Make the save.

A failed save raises Infestation by one step. Note that Infestation never carries the overreach tally with it: the tally resets to 0 on any Long Rest, but Infestation, once gained, remains until you reduce it deliberately.

The Six Steps

Each step of the Infestation Track deepens the symbiote's hold. Steps 1 and 2 are largely cosmetic; 3 and 4 give the symbiote a voice and a hand; 5 lets it override you outright; 6 is the end of you.

The Infestation Track

Reducing Infestation

The self is not so easily lost that it cannot be recovered — until the very end. Infestation falls through true rest, the ministrations of the Sporesung, and restorative magic. It can never drop below 0.

Reducing Infestation

At Step 5, ordinary rest and lesser restoration no longer suffice — only Sporesung communion or greater restoration can pull you back. Step 6 is beyond all of these; a Lost soul returns, if at all, only through a journey to the Heartweave or an intervention of legend, and that is a matter for the story, not the table's arithmetic.

Releasing, Transferring, and Death

Releasing a Symbiote. During a Short Rest or Long Rest of communion, you may willingly release a bonded symbiote, freeing a bond slot and reducing your Infestation by 1. If your Infestation is 3 or higher, the parting resists you: make a Communion saving throw. On a success you release it cleanly and gain the reduction; on a failure the symbiote clings, the bond holds, and your Infestation instead rises by 1. A symbiote does not want to be put down.

Transferring. A released symbiote is simply a living creature again, and another character may bond it during a rest of their own. Infestation is a property of the bond, not the organism — a symbiote carries no Infestation to its new host, who begins the relationship at 0, Your Own. The most cynical delvers of the Hollowed Court have built long, terrible careers on this fact.

Death. If you die while bonded, your symbiotes detach and can be recovered and re-bonded by others. But a death at high Infestation is a darker thing: if you die while at Step 5, the symbiotes may refuse to let go, “continuing” you into something of the Blight Below rather than releasing your body — the DM decides whether you rise as a Sporeborn (see Chapter 3) or are Lost as at Step 6. What the Deep loves, it is slow to surrender.

A Catalog of Symbiotes

What follows are ten sample symbiotes of roughly uncommon power, ready to hand a party found in the Velvetfields or bartered for in the Driftmarket. Each lists its appearance, its always-on Boon, and one or two Gifts with their Communion cost. Save DCs and Gift attack rolls follow the rules under Communion, above.

Reachthorn, the Living Blade

A blade of vein-threaded chitin that grows from your hand or forearm, weeping a thin, bright sap.

Boon. While Reachthorn is bonded, you can grow or dismiss the blade at will (drawing it in combat requires no action). It is a magic weapon you are proficient with, dealing 1d8 Slashing damage; it has the Finesse property and the Vex mastery, and it can't be dropped, disarmed, or sundered against your will.

Thirsting Cut (1 CP). When you hit a creature with Reachthorn, the wound drinks. The target takes an extra 1d6 Necrotic damage, and you gain Temporary Hit Points equal to that Necrotic damage.

Reaping Lash (2 CP). Bonus Action. The blade lengthens into a whip; make one melee attack with Reachthorn against a creature within 15 feet. On a hit it deals its normal damage plus an extra 1d8, and the target can't take Reactions until the start of your next turn.

The Mantle of Cilia

A cloak of pale, faintly glowing cilia that lies over your shoulders and hardens like living scale.

Boon. While you wear no armor or Light armor, your base Armor Class becomes 13 + your Dexterity modifier. The mantle sheds Dim Light in a 5-foot radius, which you can suppress.

Bloom Guard (1 CP). Reaction, when you take damage. The cilia harden over the blow. You have Resistance to that instance of damage.

Sheltering Bloom (2 CP). Bonus Action. The mantle unfurls into a canopy. You and each ally within 5 feet gain Temporary Hit Points equal to 2 + your Proficiency Bonus.

The Weeping Lash

A coil of barbed tendril at your wrist that uncoils into a whip beaded with weeping spores.

Boon. You have a magic natural weapon with Reach 10 feet that deals 1d6 Poison damage and has the Sap mastery (on a hit, the target has Disadvantage on its next attack roll before the start of your next turn).

Snaring Coil (1 CP). Bonus Action. One creature within 10 feet makes a Strength saving throw against your Symbiote save DC or is pulled up to 10 feet toward you and has the Grappled condition (escape DC equals your Symbiote save DC).

Blightburst (2 CP). Action. The barbs bloom in a 15-foot Cone. Each creature there makes a Constitution saving throw against your Symbiote save DC. On a failure, a creature takes 2d8 Poison damage and has the Poisoned condition until the end of its next turn; on a success, it takes half as much damage and no condition.

Wisplight, the Kindled Lantern

A small luminous bulb that rides your shoulder, brightening and dimming with your breath.

Boon. You shed Bright Light in a 15-foot radius and Dim Light for an additional 15 feet, in any color you choose. As a Bonus Action you can raise or lower this light, from daylight-bright to fully dark, and you see through its Dim Light as if it were Bright Light.

Rallying Glow (1 CP). Bonus Action. One creature within 30 feet of you that stands in your light gains Temporary Hit Points equal to your Proficiency Bonus and can end the Frightened condition on itself.

Beacon Flare (2 CP). Bonus Action. The bulb flares white. Each enemy within 15 feet makes a Constitution saving throw against your Symbiote save DC or has the Blinded condition until the end of your next turn, and each ally within 15 feet has Advantage on their next attack roll made before the end of your next turn.

Delvewurm, the Questing Root

A segmented, mole-soft spore-beast that nests along your spine and can slip free to scout ahead.

Boon. You have a Burrow Speed of 10 feet through loose earth, loam, and fungus, and you have Advantage on Wisdom (Perception) checks that rely on scent or vibration.

Send the Scout (1 CP). Bonus Action. The beast detaches for up to 10 minutes as a Tiny scout (Speed 30 feet, Climb 30 feet, Burrow 10 feet) that you can see and hear through; while you do, you are Blinded and Deafened to your own surroundings. It returns to you as a Bonus Action, or reforms on you after a Long Rest if destroyed.

Underreach (2 CP). Bonus Action. You burrow through loose ground and resurface in an unoccupied space within 30 feet that you can see or sense. This movement doesn't provoke Opportunity Attacks.

Mendcap, the Knitting Bloom

A soft-gilled bloom at your sternum that opens to breathe a warm, green-gold haze.

Boon. At the start of each of your turns while you have 0 Hit Points, you automatically stabilize.

Knit (1 CP). Bonus Action. You touch a creature (or yourself), and it regains 1d8 + your Proficiency Bonus Hit Points.

Blooming Renewal (3 CP). Action. You touch a creature; it regains 4d8 + your Proficiency Bonus Hit Points, and you end one of the following conditions on it: Blinded, Deafened, Paralyzed, Poisoned, or Stunned. You may instead divide the healing (rolled once) among up to three creatures within 10 feet of you.

Chitinward, the Bulwark

Overlapping plates of dark fungal chitin that lock over you like a beetle's shell.

Boon. You gain a +1 bonus to Armor Class, and you have Advantage on saving throws against being knocked Prone or moved against your will.

Spine Riposte (1 CP). Reaction, when a creature within 5 feet hits you with an attack. Barbs erupt from the carapace, and the attacker takes 1d8 Piercing damage.

Fortress Bloom (2 CP). Bonus Action. You root and harden until the start of your next turn: you gain a +3 bonus to Armor Class and can't be moved against your will, but your Speed becomes 0.

Choruscap, the Listening Bloom

A fringe of small, ear-like gills that unfurls at your throat and temples and turns toward sound.

Boon. You can speak telepathically to any creature within 30 feet that shares a language with you, and you have Advantage on saving throws against being Charmed or Frightened.

Borrowed Sense (1 CP). Bonus Action. One willing creature within 30 feet shares its senses with you until the start of your next turn: you can see and hear through it, or read its surface thoughts.

Dissonant Spore (2 CP). Action. One creature within 30 feet that can hear you makes a Wisdom saving throw against your Symbiote save DC. On a failure it takes 3d6 Psychic damage and must immediately use its Reaction, if it has one available, to move as far from you as its Speed allows.

Driftsail, the Silken Sail

Translucent spore-silk membranes that unfurl between arm and flank and catch the cavern-drafts.

Boon. You take no damage from falling and can glide, moving up to 20 feet horizontally for every 10 feet you fall. You also have Advantage on Dexterity (Acrobatics) checks made to keep your footing.

Sudden Leap (1 CP). Bonus Action. Your jump distance is tripled until the end of your turn, and your movement doesn't provoke Opportunity Attacks this turn.

Riding the Drift (1 CP). Reaction, when you fall, are moved against your will, or are missed by a melee attack. You glide up to 20 feet to an unoccupied space you can reach.

Emberpuff, the Kindling Bloom

A cluster of swollen, ember-lit puffballs across your shoulders that smolder when your blood is up.

Boon. As a Bonus Action you can hurl a spore-ember at a creature or object within 60 feet: make a ranged attack roll using your Proficiency Bonus + your Constitution modifier. On a hit, the target takes 1d6 Fire damage. This damage increases to 2d6 when you reach 11th level.

Searing Puff (2 CP). Action. Caustic spores gust in a 15-foot Cone. Each creature there makes a Dexterity saving throw against your Symbiote save DC, taking 2d6 Fire damage on a failure or half as much on a success.

Rupture (3 CP). Action. Choose a point within 60 feet; the spores detonate in a 15-foot-radius Sphere. Each creature there makes a Constitution saving throw against your Symbiote save DC, taking 4d6 Fire damage on a failure or half as much on a success. A creature that fails also can't regain Hit Points until the start of your next turn.

Symbiosis for the Dungeon Master

Symbiosis is the promise this book keeps and the price it charges, and it belongs to you to pace. Handled well, it is the truest expression of the Deep at your table: power that loves the one who wields it, a little too much to be safe.

When to Offer the First Symbiote. Let the Velvetfields make the offer. A single, gentle symbiote — Wisplight, or Mendcap, or the Mantle of Cilia — pressed into a player's hands by a myconid shepherd in the first session or two teaches the whole system without endangering anyone. Introduce Communion before you ever mention Infestation. Let the party feel the gift as a gift first. The teeth can wait for the second reach.

Infestation as Story, Not Punishment. A failed Communion save is not a slap; it is a plot advancing. The Whisper and the Bloom cost almost nothing mechanically, on purpose — they exist so the fiction moves first. Voice the symbiote in the quiet after a hard fight. Let the veins climb a favorite character's throat one campaign-night at a time, so that when the Reaching finally turns their own hand against them, it lands as tragedy rather than bookkeeping.

Pacing the Pressure. Blight exposure is your dial. In the safe Reaches, Communion saves come only when players overreach or fall, and symbiosis feels generous, as it should. In the Bruise, near the Rustgyre, or wherever the Blight Below presses close, the saves multiply and the party's borrowed power turns liability exactly where they most want to lean on it. The Deep is most beautiful where it is safe to say yes, and most hungry where saying yes will cost you everything.