Aknudag the Deep Sea Titan

Cud Chum. If the titan starts its turn with its Acid Glob already charged and it is currently swallowing one or more Medium or Small creatures, it must make a DC 15 Wisdom saving throw. On a failed save, it is forced to use Acid Glob as its action for this turn unless it is somehow incapable of doing so. It may choose the target as normal.

Dazzling Bioluminescence.  The titan sheds dim light in a 0 to 20 foot radius. The titan can alter the size of the radius as a bonus action. Any creature with a natural swimming speed that starts its turn within this radius or enters the area for the first time on a turn must succeed on a DC 17 Wisdom saving throw. On a failed save, it is charmed by the titan until it is no longer illuminated by the light or until it takes damage. A creature charmed in this way is also incapacitated. A creature that succeeds on this saving throw or has the effect end for it is immune to the effect for the next 24 hours. Vampire Squids automatically fail this saving throw.

Digestive Regeneration. At the start of each of its turns, the titan deals 25 (10d4) acid damage to any creatures it has swallowed and regains hit points equal to half the combined damage taken by all swallowed creatures. If it causes a creature to fail a death saving throw, it regains a number of hit points equal to that creature's number of hit dice.

Innate Spellcasting. The titan's spellcasting ability is Wisdom (spell save DC 17). It can cast the following spells, requiring no material components:

3/day each: Command (3rd-level version), Dispel Magic, Hunter's Mark (3rd-level version)

Legendary Resistance (2/day). If the titan fails a saving throw, it can choose to succeed instead.

Limited Amphibiousness. The titan can breathe air and water, but it needs to be submerged at least once every 4 hours to avoid suffocating.

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

Bite. Melee Weapon Attack: +11 to hit, reach 10 ft., one target. Hit: 17 (3d6 + 7) piercing damage plus 5 (2d4) acid damage. If the target is a creature, it must succeed on a DC 19 Strength saving throw or be grappled (escape DC 19). While grappled in this way, it is also restrained. If the target is coated in acid from the titan's Acid Glob action, this attack has advantage.

Swallow. The titan makes a bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is also swallowed, and the grapple ends. While swallowed, the target is blinded and restrained, it has total cover against attacks and other effects outside the titan, and it takes acid damage at the start of each of the titan's turns as described in digestive regeneration. The titan can have up to two Small or Medium creatures swallowed at a time. 

If the titan takes 30 damage or more on a single turn from a swallowed creature, it immediately recharges its Acid Glob.

If the titan dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 15 feet of movement, exiting prone.

Claw. Melee Weapon Attack: +11 to hit, reach 15 ft., one target. Hit: 14 (3d4 + 7) slashing damage. This attack deals double damage to objects and structures.

Squid Launcher (2/day). The titan launches a Vampire Squad at a creature it can see within 120 feet of itself. The target must make a DC 19 Dexterity saving throw. On a failed save, the attaches to it. On a successful save, the squid appears at an unoccupied space within 5 feet of it. A Vampire Squid uses the statistics of a Stirge with the following changes:

• It can breathe only underwater or while attached to a creature.
• It replaces its flying speed with a swimming speed.
• It has advantage on melee attack rolls against any creature that doesn't have all its hit points.
• If forcibly detached from a creature, it can use its reaction to immediately make a Blood Drain attack against the creature that detached it.
• It is immune to all damage except psychic damage while attached to a target and when it would take damage it's immune to, the target takes it instead.
• It does not detach from its prey until the prey is dead.
• For each turn after the first that it remains attached to a target, the damage from Blood Loss increases by 1d4.

Acid Glob (Recharge 5-6). The titan spits a glob of acidic mucus at a point it can see within 60 feet of itself. Each creature within 20 feet of that point must make a DC 19 Dexterity saving throw. On a failed save, it takes 20d4 acid damage and is coated in acid. A creature coated in acid takes 5d4 acid damage at the end of each of its turns and has its movement speed halved. The acid remains until the target or another creature within 5 feet of it uses an action to scrape it off. Alternatively, if a creature spends its entire turn fully submerged underwater, the acid is washed away at the end of the turn (after dealing its damage). On a successful save, a creature takes half as much damage and is not coated in acid.

If the titan has any swallowed creatures when it uses this action, they are expelled and land in the center of the acid glob. They take 1d6 bludgeoning damage for every 10 feet away from the titan they land, or half as much damage if they land underwater, and must make the Dexterity saving throw for Acid Glob as one of its targets.

Silent Swimmer. While underwater, the titan can attempt to hide as a bonus action.

Ambush Predator. When a creature enters the reach of the titan's bite, it can use its reaction to make a bite attack against that creature.

The titan 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. The titan regains spent legendary actions at the start of its turn.

Attack. The titan makes a claw attack or uses its squid launcher.

Move. The titan moves up to half its speed.

Chomp (Costs 2 Actions). The titan makes a bite attack or uses its swallow.