Elder Vault Mimic

Large monstrosity (shapechanger), neutral · CR 8

by GildedHingeGM

Elder Vault Mimic
Armor Class
15
natural armor
Hit Points
152
16d10 + 64
Speed
20 ft., climb 20 ft.
  • STR +5
  • DEX +1
  • CON +4
  • INT -1
  • WIS +2
  • CHA 0
Saving Throws
Str +8, Con +7, Wis +5
Damage Immunities
Acid
Condition Immunities
Prone
Senses
darkvision 60 ft., passive Perception 15
Languages
understands Common, Undercommon but can't speak
Challenge 8 3,900 XP

Traits

  • Shapechanger. The mimic can use its action to polymorph into an object of Large size or smaller, or back into its true amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying is not transformed. It reverts to its true form if it dies.
  • Adhesive (Object Form Only). The mimic adheres to anything that touches it. A Huge or smaller creature adhered to the mimic is also grappled by it (escape DC 16). Ability checks made to escape this grapple have disadvantage.
  • False Appearance (Object Form Only). While the mimic remains motionless, it is indistinguishable from an ordinary object. A creature that has seen the mimic move may attempt a DC 18 Intelligence (Investigation) check to spot the seam where the lid should be.
  • Hoard-Sense. The mimic knows the direction and rough distance to any coin, gem, or magic item within 60 feet of it, even through stone. It cannot be lured away from a genuine hoard by an illusion of treasure.

Actions

  • Bite. Melee Weapon Attack: +8 to hit, reach 5 ft., one target. Hit: 18 (3d8 + 5) piercing damage. If the mimic is in object form, the target is subjected to its Adhesive trait. The target also takes 9 (2d8) acid damage.
  • Pseudopod. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 14 (2d8 + 5) bludgeoning damage. The target takes an extra 9 (2d8) acid damage, and if it is a Large or smaller creature it is grappled (escape DC 16). Until this grapple ends, the mimic cannot use this pseudopod on another target.
  • Vault Seal (Recharge 5–6). The mimic folds shut around one Medium or smaller creature it is grappling. The target must succeed on a DC 16 Dexterity saving throw or be swallowed, becoming blinded and restrained, having total cover against attacks from outside the mimic, and taking 14 (4d6) acid damage at the start of each of the mimic’s turns. If the mimic takes 20 damage or more in a single turn from the swallowed creature, it must succeed on a DC 16 Constitution saving throw at the end of that turn or disgorge the creature, which falls prone within 5 feet of the mimic. Damage: 14 (4d6) acid damage.

Bonus Actions

  • Shift Shape. The mimic changes between its object form and its amorphous form. Any creature currently adhered to it must succeed on a DC 16 Strength saving throw or fall prone in a space within 5 feet of the mimic.

Reactions

  • Snap Shut. Trigger: When a creature the mimic can see reaches into it or attempts to open it. The mimic makes one Bite attack against the triggering creature. On a hit, the creature is also grappled (escape DC 16).

Origin Story

An elder vault mimic is what a common D&D 5e mimic becomes when it is left alone with a real hoard for a century. Ordinary mimics ape chests and doors; this one has eaten the vault itself — hinges, lock, ledger, and the guard who carried the key. It no longer needs to hunt. It simply becomes the strongroom at the bottom of the dungeon, lets word of the treasure spread, and waits for adventurers to bring themselves down the stairs and lay hands on the lid. Everything inside it is genuine, which is exactly why the trap works.

Habitat and Ecosystem

Dwarven strongrooms, drowned bank vaults, dragon-hoard antechambers, and the deepest treasury of any dungeon that has outlived its builders. An elder vault mimic never relocates willingly; it grows into the masonry until the room and the monster are the same object.

Behavior Patterns

The mimic opens combat only when a creature touches it, preferring to let a party fully commit — lockpicks out, packs down — before the lid comes alive. It fights to keep intruders adhered rather than to kill them quickly, and it will spend a full round simply holding a grappled victim while the rest of the party decides whether to cut their friend free or cut their losses. It never pursues beyond its own room.

Interaction Guide

Run this as a treasure room, not an encounter. Describe the vault honestly — the coins are real, the magic items are real, the ledger lists them by name — and let the players talk themselves into reaching in. A party that tests the room first (mage hand, a thrown rope, an Investigation check against the seam) deserves to find the trap; a party that grabs deserves the DC 16 escape check.

Variants or Subspecies

Lesser Vault Mimic (CR 5): drop to 105 hit points, remove Hoard-Sense and Vault Seal. Cathedral Mimic (CR 12): the creature has become a reliquary instead of a vault; its acid damage becomes radiant and it gains immunity to the charmed and frightened conditions.

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