Cannoloth Abomination

Multiattack. The Canoloth makes either an attack with its tongue and bite attacks or makes 4 attacks with its tentacles.

Dimensional Lock. Other creatures cant teleport within 60 feet of the Canoloth. Any attempt to do so is wasted

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

Magic Weapons. The canoloths weapons are magical.

Bite.  Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 54 (14d6 + 5) piercing damage. 

Tentacle Whip. Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 32 (5d10 + 5) bludgeoning damage. If the target is large or smaller they are grappled (escape DC 19). For every 20 damage it takes on its turn the Canoloth releases one tentacle

Tongue. Melee Weapon Attack: +10 to hit, range 30 ft., one target. Hit: 37 (5d12 + 5) [damage type] damage. If the target is Medium or smaller, it is grappled(escape DC 19), pulled up to 30 feet toward the canoloth, and is restrained until the grapple ends. The canoloth can grapple one target at a time with its tongue.

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Tear Asunder (Costs 2 Actions). If the canoloth has a target held by 2 or more tentacles, it can use this action to begin ripping the target apart. The target must make a Con Save DC 19, taking half on a successful save.

2 Tentacles: 26 (4d12)

3 Tentacles: 39 (6d12)

4 Tentacles: 65 (10d12)

Tentacle Whip. (Costs 1 Action) Melee Weapon Attack: +10 to hit, reach 15 ft., one target. Hit: 32 (5d10 + 5) bludgeoning damage. If the target is large or smaller they are grappled (escape DC 19). For every 20 damage it takes on its turn the Canoloth releases one tentacle

When the Abomination reaches 0 hp it opens its back revealing a gigantic hidden mouth. Its hitpoints are reset to max and the creature loses all resistances. 

It loses its normal bite attack but can use its tongue and tentacles in its attacks. if the tongue strikes a creature it will pull the creature in to the abominations maw. The creature must make a strength or dex save (DC 18). On a fail they take 68 (14d8+5) piercing damage.