Amphibious. The kragen can breathe air and water.

Freedom of Movement. The kragen ignores difficult terrain, and magical effects can't reduce its speed or cause it to be restrained. It can spend 5 feet of movement to escape from nonmagical restraints or being grappled.

Multi-head.  The kragen has advantage on Wisdom (Perception) checks and on saving throws against being blinded, charmed, deafened, frightened, stunned and knocked unconscious. For each head the kraken has beyond one, it gets an extra reaction that can be used only for opportunity attacks.

Siege Monster. The kragen deals double damage to objects and structures.

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

Multiattack. The kragon can use its Frightful Presence. It then makes three attacks: any combination of bite, tail, tentacles and claw attacks. 

Bite. Melee Weapon Attack: +17 to hit, reach 5 ft., one target. Hit: 23 (3d8 + 10) piercing damage. If the target is a Large or smaller creature grappled by the kraken, that creature is swallowed, and the grapple ends. While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the kraken, and it takes 42 (12d6) acid damage at the start of each of the kraken's turns.

Claw. Melee Weapon Attack: +16 to hit, reach 10 ft., one target. Hit: 16 (2d6 + 9) slashing damage.

If the kragen takes 50 damage or more on a single turn from a creature inside it, the kragen must succeed on a DC 25 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of the kragen. If the kragen dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 15 feet of movement, exiting prone.

Tentacle. Melee Weapon Attack: +17 to hit, reach 30 ft., one target. Hit: 20 (3d6 + 10) bludgeoning damage, and the target is grappled (escape DC 18). Until this grapple ends, the target is restrained. The kraken has ten tentacles, each of which can grapple one target.

Tail. Melee Weapon Attack: +16 to hit, reach 20 ft., one target. Hit: 18 (2d8 + 9) bludgeoning damage.

Fling. One Large or smaller object held or creature grappled by the kragen is thrown up to 60 feet in a random direction and knocked prone. If a thrown target strikes a solid surface, the target takes 3 (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 saving throw or take the same damage and be knocked prone.

Lightning Storm. The kragen magically creates three bolts of lightning, each of which can strike a target the kraken can see within 120 feet of it. A target must make a DC 23 Dexterity saving throw, taking 22 (4d10) lightning damage on a failed save, or half as much damage on a successful one.

Lightning Breath (Recharge 4–6). The kragen exhales lightning in a 120-­-foot line that is 10 feet wide. Each creature in that line must make a DC 23 Dexterity saving throw, taking 88 (16d10) lightning damage on a failed save, or half as much damage on a successful one.

Frightful Presence. Each creature of the kragen's choice that is within 120 feet of the kraken and aware of it must succeed on a DC 19 Wisdom saving throw or become frightened for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a creature’s saving throw is successful or the effect ends for it, the creature is immune to the kraken's Frightful Presence for the next 24 hours.

The kragen can take 4 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 kraken regains spent legendary actions at the start of its turn.

Detect (Costs 1 Action). The kragen makes a Wisdom (Perception) check.

Tentacle, Bite, Claw, Tail Attack or Fling. The kragen makes one attack or uses its Fling.

Lightning Storm (Costs 2 Actions). The kragen uses Lightning Storm.

Lightning Breath (Recharge 4–6). The kragen exhales lightning in a 120-­-foot line that is 10 feet wide. Each creature in that line must make a DC 23 Dexterity saving throw, taking 88 (16d10) lightning damage on a failed save, or half as much damage on a successful one.

Ink Cloud (Costs 3 Actions). While underwater, the kragen expels an ink cloud in a 60-foot radius. The cloud spreads around corners, and that area is heavily obscured to creatures other than the kraken. Each creature other than the kraken that ends its turn there must succeed on a DC 23 Constitution saving throw, taking 16 (3d10) poison damage on a failed save, or half as much damage on a successful one. A strong current disperses the cloud, which otherwise disappears at the end of the kraken's next turn.