Shambling Mound

A shambling mound ally is a druid's bound guardian, a hag's pet on temporary loan, or a fey-touched grove protector that has decided the party is on the right side of an ongoing dispute. The mound does not speak (no languages) and does not understand abstract instructions. It responds to scent markers, druidcraft signals, or a fey patron's commands, and it will do exactly one thing per scene: defend this hut, drown that enemy in the bog, drag this prisoner back to the cave.

In combat, the mound is a tank and a control piece in one body. 110 HP and AC 15 absorb hits the party cannot. Three Charged Tendril attacks at +7 for 12 each pull enemies into reach, and Engulf locks a single enemy out of the fight entirely while dealing 10 lightning per round inside. Point it at the enemy spellcaster, let it grapple, and the rest of the encounter is the party cleaning up mooks while the wizard suffocates in mulch.

Lightning Absorption makes the mound the perfect tank in front of a friendly storm sorcerer or tempest cleric. Cast Lightning Bolt through the mound at an enemy line, and the mound heals from the friendly fire while the enemies take full damage. This is a real combo and worth setting up explicitly with the player. Resistance to Cold and Fire and immunity to Lightning, Deafened, and Exhaustion mean the mound stays in the fight long after a comparable CR 5 brute would be down.

When the named task is done, the mound walks back into the swamp and lies down. Do not narrate a goodbye. The mound is not sentient enough to have one, and the absence of farewell sells the strangeness of the ally better than any line.

A shambling mound is the swamp encounter that punishes lightning casters and grappler-shy parties at the same time. It looks like a pile of wet vegetation until it stands up, and Stealth +3 plus a Passive Perception of 10 means it survives by being mistaken for terrain. Stage the encounter with the mound already in cover (a riverbank, a barrow door, the edge of the camp), and let the first attack come when a PC steps into reach. The shape of the threat is the surprise round.

Lead with Multiattack: three Charged Tendril attacks at +7 for 12 average damage each, and Medium or smaller targets are pulled 5 ft. toward the mound on a hit. That pull stacks across three attacks, so a wizard at 15 ft. is in melee on round one whether she planned to be or not. Replace one tendril with Engulf when a Medium or smaller target is within 5 ft.: DC 15 Strength save, fail means Grappled (escape DC 14), Blinded, Restrained, and 10 lightning damage at the start of every turn. Engulf one PC at a time and the rest of the party has to choose between killing the mound and freeing their friend.

Lightning Absorption is the trap. A storm sorcerer or tempest cleric who opens with Lightning Bolt heals the mound for the full damage rolled, and the table reaction when 28 HP comes back to a CR 5 brute is a memorable lesson. Telegraph the trait by having the mound stand under an active thunderstorm, dripping and unbothered, before initiative. Resistance to Cold and Fire means the obvious area damage spells are also weaker than they look. Force, radiant, and slashing are the answer, and a party that figures that out mid-fight feels clever.

Run the engulfed PC's turns differently. Describe rotting plant matter pressing into their mouth, the lightning stabbing through the wet, and ask them what they are trying to do before you ask for the escape roll. The fight runs long when one party member is trapped, and that long fight is the point.