INSIDE:
Debilitating Aura. Once aware of where they are, and while inside the Colossal Mimic, creatures that are not immune to psychic damage, take 1d12 +8(per round of combat) psychic damage.
OUTSIDE:
Legendary Resistance (4/Day). If the Colossal Mimic fails a saving throw, it can choose to succeed instead.
Siege Monster. The Colossal Mimic deals double damage to objects and fellow structures.
Unusual Nature. The Colossal Mimic doesn’t require air or sleep.
INSIDE:
Acidic Digestion. Melee Weapon Attack: +25 to hit, All touching a surface inside (flying, hovering, or otherwise immaterial creatures need not apply). Hit: 12 (1d8 + 8) Acid damage. If the target is not immunde, it is poisoned (recover DC 18).
OUTSIDE:
Multiattack. The Colossal Mimic makes one Bite attack, two Tendril attacks, and one Acid Spit attack.
Bite. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 19 (2d10 + 8) piercing damage plus 11 (2d10) psychic damage.
Tendril. Melee Weapon Attack: +15 to hit, reach 15 ft., one target. Hit: 11 (1d6 + 8) slashing damage.
Acid Spit. Melee Weapon Attack: +15 to hit, reach 15 ft., one creature. Hit: 12 (1d8 + 8) psychic damage. If the target is not immune, it is poisoned (recover DC 18).
Acid Breath (Recharge 5–6). The Colossal Mimic exhales Acid Mist in a 120-foot line that is 15 feet wide. Each creature in that area must make a DC 22 Constitution saving throw, taking 55 (10d10) Acid damage on a failed save, or half as much damage on a successful one. On a success or failure, if the creature isn’t immune, it becomes Severely Poisoned.
While Severely Poisoned, the creature takes 16 (3d10) psychic damage at the start of each of its turns. The creature can repeat the saving throw at the end of each of its turns, ending the effect on itself after it succeeds on three of these saves. If the creature is targeted by magic that ends a curse or restores 40 hit points or more, the Severe Poisoning condition ends.
If a Humanoid is reduced to 0 hit points while Severely Poisoned, the creature is stable but remains unconscious for 6d12 hours. When the period of unconsciousness ends, the creature transforms into a giant mimic with all its hit points. Casting a wish spell on the unconscious creature rids it of the Severe Poisoning and prevents it from turning into a Giant Mimic.
As a bonus action, the Colossal Mimic can psychically compel (DC 22 Intelligence Saving Throw) one creature to use their movement on their next turn to move within Melee range (15 feet) of the Colossal Mimic and remain there until their following turn. If they do not have enough movement speed, they must take the dash action to get within melee range.
As a reaction, The Colossal Mimic can make an additional bite attack on a creature within melee range that has both succeeded a saving throw and attacked the Colossal Mimic in this round of combat.
The Colossal Mimic can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn. The dragon regains spent legendary actions at the start of its turn.
Tentacle. The Colossal Mimic makes one Tentacle attack.
Shatter Concentration (Costs 2 Actions). The Colossal Mimic targets a creature it has poisoned. The target’s concentration on a spell it has cast or an ability it is maintaining ends, and the target takes 19 (3d12) psychic damage.
Description
For the purposes of the campaign session in which I'm using this monster, I've written parts of this description to account for a previous BBEG's powers and disappearance, feel free to omit those parts or adapt them to your usage. The Colossal Mimic is loosely based upon the Elder Brain Dragon in Fizban's Treasury of Dragons, but is a "What if the Elder Brain took over a mimic instead of a dragon?" scenario.
A Mansion on top of a hill, overlooking the nearby town, but at a distance far enough away that nothing can be heard for miles. No wildlife appears to be bustling in the surrounding shrubbery, no birds or bugs, nor any sign of life beyond you and your travelling companions.
Upon initial discovery, the Colossal Mimic does not appear to be a monster. Instead, it appears as a Mansion that at one point probably appeared to be of the highest class. It has now been upkept with perfunctory effort. The outside needing a coat of paint. The inside dusty and unkempt. The window trimmings and wallpaper all of dated design. The hardwood floor is replete with dust, dirt, and muck that has filled every crack and crevice in the ornate hardwood parkay flooring.
**Reveal Description: From within, the walls of the mansion begin to sweat. A viscous film begins to form on nearly all surfaces you can see. The floor, now soft and spongy, gives beneath your feet. The once hardened surfaces are now covered in fat and sinew.
As the Illusory magic continues to wear off, you feel woozy as you realize that you're inside something. The walls heave ever so slightly as they shift and reorient themselves to accommodate the prey within.**
No one knows the Colossal Mimic exists. This creature has never been seen before, presumably because no one has ever survived seeing one. As such, there isn't any known origin for said creature.
DC 24 History check would reveal that years ago there were rumors of a rogue force corrupted with volatile chaos magic within an Illithid colony that splintered off, kidnapped the elder brain, and made a hasty getaway. Their nautiloid spelljammer was rumored to have crashed in this general area, but no crash, debris, or Illithid (mind flayers) were ever found. Most of the nearby populace have long since written off such rumors as fear mongering to distract from the evil forces that still plague the region.
DC 22 Insight Check following the above history check:
It seems likely that the elder brain housed within this monstrosity was likely pulling the strings for Kolgar, fueling his chaotic magics with it's own, and amplifying his power.
Lair and Lair Actions
INSIDE:
On Initiative 20 (Losing Ties) the following effect happens.
Acid Splash: Stomach acid sloshes around freely inside of the mimic's insides, eating away at anything inside. As the Acid splashes around, everyone inside takes (3d12) Acid damage. +8 psychic Damage
OUTSIDE:
While Rooted, the Colossal Mimic is considered in it's Lair. A rooted Colossal Mimic has no movement speed.
Flip a coin. On Initiative 20 (Losing Ties) one of the following effects happen.
Ground Tremor (Tails). The Colossal Mimic tugs at it's roots, causing the entire surrounding area to shake violently, all within 60 ft must make a strength saving throw DC 22 or be knocked prone.
Psionic Wave (heads). A Pulse of Psychic energy emanates from the Colossal Mimic's brain, all within 80 ft of the brain must make a DC 22 Wisdom Saving throw or be at disadvantage on all checks, saving throws, and attack rolls until the following round.
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