Purple Worm

A purple worm ally is a hard sell, and the framings that work are narrow. A drow priestess has loaned the party a worm she controls through a relic, the party has acquired an artifact that briefly directs one specific worm, or a wizard has bound the worm with a high-tier conjuration for a single scene. The worm is Int 1, has no language, and does not understand commands beyond very basic directional pushes. Treat it as a guided missile with a one-minute fuel tank.

The job is structural demolition. Tunneler at half burrow speed through solid rock, leaving a 10-foot tunnel, is the kind of ability that breaks a siege, opens a sealed vault, or drops the floor of a fortress into the cavern below it. Point the worm at a wall and let it work. In combat the worm's value is the Bite-Swallow combo against a single high-value target the party wants removed from the field: the enemy general, the lich's body before it can phylactery-jump, the giant who is one round from killing the rogue. Direct it at one creature, accept that the worm will not coordinate beyond that, and get the party out of its 10-foot reach because the worm cannot tell friend from food.

The worm leaves on its own schedule. Whatever binding holds it ends in a minute or two of game time, and the moment it ends the worm decides whether to eat the binder, eat the party, or burrow off into the deep dark. Players who keep their distance and have an exit plan ready will see it tunnel away. Players who got greedy will see what the adversary version of this stat block does at point-blank range.

Have the binding artifact crack visibly when it is about to fail. Players who see the warning and act on it earn a clean exit. Players who ignore it earn the encounter they thought they were avoiding.

A purple worm is the Underdark's answer to weather. CR 15, AC 18, 247 HP, walk 50 ft., burrow 50 ft., Tunneler carves through solid rock at half burrow speed leaving a 10-foot tunnel. Tremorsense 60 ft. plus Blindsight 30 ft. means the worm knows where the party is the moment they step on stone, so there is no surprising it from the ground.

The opener is the burst. The worm erupts out of the floor in the middle of the marching order, which puts every PC within 10-foot reach of the Bite. Bite is +14 for 22 piercing and grapples Large or smaller targets on a hit (escape DC 19), with Restrained until the grapple ends. Tail Stinger is a separate +14 attack for 16 piercing plus 35 poison, which on an average roll one-shots most mid-tier squishies. Multiattack is one Bite plus one Tail Stinger. Round one: Bite the front line and grapple, Tail Stinger the cleric in the back row, then bonus action Swallow on the grappled PC (DC 19 Strength). Inside the worm: Blinded, Restrained, Total Cover from outside, 17 acid at the start of each of the worm's turns, up to three creatures at once.

The swallow rule is the encounter's clock. If the party deals 30+ damage from inside in a single turn, the worm makes a DC 21 Con save or regurgitates everyone Prone within 5 feet. So the swallowed PC's loop is: hit hard, hope the worm fails, get spat out. The party outside is racing against acid damage with 247 HP between them and rescue. Let the cleric in the gut decide between healing themselves and continuing to attack.

The worm does not flee. Int 1, no self-preservation. If it is taking too much damage in one spot, it burrows into a wall, comes out elsewhere on the map, and starts again. Use the tunnel. The 10-foot bore means the party can chase if they survive, and the worm's lair is wherever it last stopped.