Violet Fungus

A violet fungus as ally is the kind of premise a myconid sovereign offers, a druid PC cultivates in their grove, or a necromancer NPC hands the party as a hedge against intruders in their hideout. It is mindless. It does not befriend, it does not respond to commands beyond the basic "stay put" of being planted somewhere, and it cannot move meaningfully (5 ft. walk speed). Treat it as a defensive installation, not a companion.

Where it earns its keep is as a passive trap. Plant a violet fungus across a doorway, in the dark corner of the party's safehouse, or behind the chest the rogue is sure no one will steal. Anything that approaches within 10 ft. eats two Rotting Touch attacks at +2 for 4 necrotic each, and the immunities to Blinded, Charmed, Deafened, and Frightened mean most spellcasting infiltrators can't disable it the easy way. Blindsight 30 ft. ignores invisibility within range, which is the actual selling point. A violet fungus in the front hall of a wizard's tower is a real reason for invisible scouts to think twice.

The cost is everything else. The fungus has AC 5 and 18 HP, so anyone with a torch and a longsword turns it into compost in two rounds. It will attack the party if the party walks past it forgetting which corner they planted it in, since it cannot tell friend from foe. Necrotic damage on the cleric who forgot is going to come up at the table and the cleric's player is going to be annoyed. That's the fungus.

Mark its location on the party map in something other than ink. The day a player erases the marker by accident is the day the fungus eats them.

A violet fungus is the Underdark's tripwire: shrub-tall, blind to light, and easy to mistake for a normal patch of mushrooms until something steps within reach. Use it as terrain that bites. The whole point is the moment a player says "I walk past the purple patch" and you ask them for initiative.

The numbers are forgiving and that is by design. AC 5, 18 HP, walk 5 ft., Blindsight 30 ft., and a single Multiattack of two Rotting Touch attacks at +2 for 4 necrotic each. The fungus will hit a level 3 PC roughly half the time and chip 8 average damage per round if both land. Run it as a hazard the party clears in two rounds, not a boss. The interesting wrinkles are the immunities to Blinded, Charmed, Deafened, and Frightened, which mean the bard cannot fast-talk it and the warlock cannot scare it. It is plant matter responding to vibration. Tell the player whose spell wasted itself, because the lesson is "this is not the right tool" and they should pivot.

Cluster them. A single fungus is a speed bump; four arranged across a 30-foot corridor are a tactical problem because the reach 10 ft. on Rotting Touch covers the whole walkway and the necrotic damage stacks. Mix them with shriekers (a different stat block) and the patch becomes an alarm-and-attack combination that draws bigger Underdark predators within minutes. The fungus does not retreat, does not pursue past 5 ft. of movement, and does not communicate. It just stands there and rots whatever brushes it.

Have the patch be growing on a corpse the party recognizes from a previous session. The horror is the recognition, not the damage roll.