Homebrew
Amphibious. The squid can breathe air and water. Underwater Camouflage. The squid has advantage on Stealth checks made while underwater.
Multiattack. The terasquid makes three attacks, one with its beak and two with its tentacles. Bite. Melee Weapon Attack: +10 1d20+10 to hit, reach 5ft, one target. Hit: 11 (2d6+6) 2d6+6 piercing damage. Slap. Melee Weapon Attack: +10 1d20+10 to hit, reach 15ft, one target. Hit: 21 (4d6+6) 4d6+6 bludgeoning damage. Grab. Melee Weapon Attack: +6 1d20+6 to hit, reach 15ft, one target. Hit: 13 (2d6+2) 2d6+2 bludgeoning damage and the target is grappled (escape DC 16). Until this grapple ends, the creature is restrained. The giant squid has two tentacles, each of which can grapple one target. A held target is dropped and knocked prone if the terasquid attempts to grab with a tentacle which is already grappling a target, regardless if the attack hits.
The terasquid can take 2 legendary actions, choosing from the list below. Only one legenday action can be used at a time and only at the end of another creature’s turn. The terasquid regains spent legendary actions at the start of its turn. Fling. One large or smaller object held or creature grappled by the colossal squid is thrown up to 30ft in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (1d6) 1d6 bludgeoning damage for every 10 feet it was thrown. If the target is thrown at another creature, that creature must succeed on a DC 18 Dexterity save or take the same damage and be knocked prone. Swallow. If the terasquid has a medium or smaller creature grappled, it can swallow the creature whole, ending the grapple. It can swallow no more than three living creatures at a time. While swallowed the creature is blinded and restrained, it has total cover against attacks and other effects outside of the terasquid, and it takes 21 (6d6) 6d6 acid damage at the start of each of the terasquid’s turns. If the squid takes 15 damage or more on a single turn from creatures inside it, the squid must succeed on a DC 25 Constitution save at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10ft of the squid. It will also automatically regurgitate any creatures if it flees. If the squid dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15ft of movement, exiting prone.