Celestial Gate Guardian

Amphibious. The Guardian can breathe air and water.

Tentacle Presence.
The Guardian itself will not show its true form, and remains beneath the ship. Instead, it appears as four, horrifyingly large tentacles, allowing the Guardian to a round of initiative per tentacle on a round. When the health of the Guardian is reduced by a quarter one of the tentacles dies.

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

Aversion to Damage. If the Guardian takes more than 10 lightning damage from a single attack, it has disadvantage on attack rolls and ability checks until the end of its highest initiative tentacle's turn. Also, any grappled creatures have advantage on their saving throws against escaping the grapple and the Grapple Slam.

Multiattack. The Guardian makes 2 Slam attacks each turn.

Slam. Melee Weapon Attack: +6 to hit, reach 50ft., one target. Hit: 3d6+5 Bludgeoning Damage.

Grasping Tentacle. When a creature is hit by one of the Guardian's tentacle slams, the creature must make a DC15 Dexterity or Strength saving throw. On a failure, the Creature is grapple. Only one creature may be grappled per tentacle. A creature grapple this way may remake the saving throw as an action in order to break free.

Grapple Slam. If a creature is grappled by one of the Guardian's tentacles when it makes a slam attack, the creature must succeed on a DC15 Constitution Saving Throw. On a failed save, the creatures takes 5d6 bludgeoning damage, and is stunned into the end of the creatures next turn. On a successful save, the creature takes half damage, but is not stunned and may make an attempt to break their grapple.

Drain Force. If a tentacle is grappling a creature, it can use its turn to drain the life force of the creature. The creature must make a DC15 Wisdom saving throw. On a failure, the creature takes 3d6 force damage, and the Guardian regains the same number of hit points as temporary hit points. On a successful save, the creature may make an attack of opportunity on the Guardian regardless of if they have a reaction or not.

The Guardian 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 Guardian regains spent legendary actions at the start of its highest initiative tentacle's turn.

Slam. The Guardian makes one Slam attack with a tentacle that is not grappling a creature.

From the Depths (Costs 2 Actions). The Guardian slams the Ship from below, causing it to lift out of the water, before slamming back down. Any creature not currently grappled by a tentacle (Or creatures currently flying) must make a DC15 Dexterity saving throw, or take 3d6 bludgeoning damage, and be knocked prone.