Anantashesha

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

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

Memory Drain. Those who get within 300 feet of Anantashesha must make a DC 25 WIS check or be charmed. A creature that is charmed by Anantashesha cannot move or act and begins muttering about an endless  and cold void. The effect ends if the creature takes damage. Those who are affected by Memory Drain will struggle to recall who they are or why they are there. They can redo this save at the start of their turn. The effect lasts until the target breaks free or is removed from the radius. If a creature is attacked while charmed by Anantashesha, Anantashesha's attacks automatically hit. If a creature fails this check three times, the Crime Against Sanity effect takes place.

Crime Against Sanity. Anantashesha is a creature which despises sapience in other beings. Those who spend too much time near the creature begin to loose track of their memories, losing their own and gaining those of others as memory drain begins to become permeant. Those that suffer this effect three times are permeantly affected by the monster's Memory Drain ability.

Amnesia Jelly. Anantashesha produces a jelly from its mouth after feeding. Those that come into contact with Anantashesha's jelly forget everything that happened to them in the past 10 years.

Fulfilled Hunger. After a creature is swallowed, Anantashesha returns to its lair at the bottom of the sea, remaining dormant for 2 weeks unless harmed.

Multiattack. Anantashesha makes three attacks, two bites before finishing with a swallow.

Bite. Melee Weapon Attack: +19 to hit, reach 15ft., one target. Hit: 36 (4d12 + 15) piercing damage, and the target must make a DC 19 Constitution saving throw, taking 32 (7d6+8) poison damage on a failed save, or half as much damage on a successful one. On hit, a target must then make a save against Anantashesha's Memory Drain effect.

Swallow.  Anantashesha makes one bite attack against a Large or smaller creature it is grappling. If the attack hits, the target takes the bite’s damage, and a second bite follows before the target is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside Anantashesha, and it takes 56 (16d6) acid damage at the start of each of the Anantashesha's turns.

Constrict. Melee Weapon Attack: +19 to hit, reach 5 ft., one creature. Hit: (10d8 + 13) bludgeoning damage. The target is grappled (escape dc 25) Until this grapple ends, the creature is restrained, and Anantashesha can't constrict another target. Once grappled, the target must make an extra save against Memory Drain every turn.

Anantashesha 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. Anantashesha regains spent legendary actions at the start of its turn.

Attack. Anantashesha makes one bite attack.

Move. Anantashesha moves up to half its speed.

Chomp (Costs 2 Actions). Anantashesha makes one bite attack or uses its Swallow.