Castle-Sized Mimic

Final Defense (Mythic Trait; Recharges after a Short or Long Rest). If the mimic is reduced to 0 hit points, it doesn’t die or fall unconscious. Instead, the damage creates cracks in the walls and the gates splinter open, issuing forth eight castle defenders (use the statblock for Champion except their monster type is monstrosity), that must stay within 200 ft. of the mimic. The Castle Defenders are not affected by attacks from the mimic and are also immune to the charmed condition - they act on the same initiative as the mimic. If the mimic finishes a short or long rest, the castle defenders return inside and it fully heals. The Castle-Sized Mimic dies when all of the Castle Defenders are destroyed.

Legendary Resistance (3/Day). If the mimic fails a saving throw, it can choose to succeed instead.

Magic Resistance. The mimic has advantage on saving throws against spells and other magical effects.

Shapechanger. The mimic can use its action to polymorph into a castle-sized building or back into its true, amorphous form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.

Adhesive. The mimic adheres to anything that touches it. Any creature adhered to the mimic is also grappled by it (escape DC 22). 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 castle-sized building.

Grappler. The mimic has advantage on attack rolls against any creature grappled by it.

Multiattack. The mimic makes three attacks: two with its Pseudopods and once with its Bite.

Pseudopod. Melee Weapon Attack: +18 to hit, reach 40 ft., one target. Hit: 18 (3d6 + 8) bludgeoning damage. On a successful attack, the target is also subjected to its Adhesive trait.

Bite. Melee Weapon Attack: +18 to hit, reach 5 ft., one target. Hit: 27 (3d12 + 7) piercing damage plus 4 (1d8) acid damage. The bite attack is disabled while the Final Defense mythic trait is active.

Deadly Barrage (Recharge 5–6). The mimic makes use of the various siege engines arrayed along the top of its walls, unleashing a barrage of ballista bolts at all creatures within 600 ft. (can't hit targets within 30 ft. of it). Ranged Weapon Attack: +18 to hit. Hit: 16 (3d10) piercing damage. Any flying creatures that are hit are also knocked prone unless they make a Dexterity (Acrobatics) check (DC 22).

Castle-Sized Mimic can take 3 legendary actions, choosing from the options below. Only one legendary action option can be used at a time and only at the end of another creature’s turn. Castle-Sized Mimic regains spent legendary actions at the start of its turn.

Move. The mimic moves up to half its speed.

Smash. The mimic makes one attack with its Pseudopod.

On Troubled Ground. The mimic jumps in the air slightly, crashing to the ground with massive impact, causing all creatures on the ground within 100 ft. to be knocked prone unless they succeed a Strength saving throw (DC 23).

Unstoppable (costs 3 actions). If the mimic is affected by any of the conditions paralyzed, stunned, or unconscious, these conditions are immediately removed.

If Castle-Sized Mimic’s Final Defense mythic trait is active, it can use the options below as legendary actions.

Trouble Below. The Castle-Sized Mimic causes the earth to break and shatter under the ground of a target within 100 feet and, unless they succeed on a Strength (Athletics) check (DC 23), they tumble into a 30 feet deep pit, taking 5d6 bludgeoning damage.

Constant Barrage (Costs 2 Actions). The Castle-Sized Mimic recharges its Deadly Barrage feature and uses it immediately.