Caliborne (Daemon Prince)

Tunneler. Caliborne can burrow through solid material at half its burrow speed and leaves a 20-foot-diameter tunnel in its wake.

Amphibious: Caliborne can breathe air and water.

Freedom of Movement: Caliborne 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.

Siren song. Caliborne can speak telepathically with Sirens of the Styx, and is able to control the actions of any Siren within 200ft of itself. This ability does not work within fields of antimagic.

Siege Monster. Caliborne deals double damage to objects and structures.

Daemon Prince. Unless its soul is trapped or destroyed, Caliborne will reform the night after it dies.

Vorpal Defiance. Caliborne's soul cannot be digested by an Icortian Vorpal Weapon.

Weak Point. Caliborne's eye has an armour class of zero. Attacks aimed at the eye deal critical damage, and make Caliborne prone. Caliborne will automatically stop being prone upon receiving up to 2 additional attacks aimed towards its eye.

Multiattack. Caliborne may make 2 actions: one with its bite, and one with its tentacles. If a creature is grappled with Caliborne's tentacles, it may instead make 3 actions, one with it's bite, one with its tentacles and one with either discharge, or regenerate.

Bite. Melee Weapon Attack: +15 to hit, reach 10 ft., one target. Hit: 22 (3d8 + 9) piercing damage. If the target is a Large or smaller creature, it must succeed on a DC 26 Dexterity saving throw or be swallowed by Caliborne. A swallowed creature is blinded and restrained, it has total cover against attacks and other effects outside Caliborne, and it takes 28 (8d6) acid damage at the start of each of Caliborne's turns.

If Caliborne takes 50 damage or more on a single turn from a creature inside it, Caliborne must succeed on a DC 21 Constitution saving throw at the end of that turn or regurgitate all swallowed creatures, which fall prone in a space within 10 feet of Caliborne. If Caliborne dies, a swallowed creature is no longer restrained by it and can escape from the corpse by using 20 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 24). Until this grapple ends, the target is Restrained, and takes 15 (3d8) Lightning damage per turn. Damage dealt to grappled targets is added to Caliborne's hit points. Caliborne has 18 tentacles, each of which can grapple one target. 

Regenerate. Caliborne regains 15d10 hit points. Lost limbs do not regrow, and scars may be left over. This ability does not work if Caliborne is prone.

Discharge. Caliborne sacrifices up to 100 hitpoints, creating a blast of energy in a 55ft radius around itself, dealing 1d10 lightning damage. Starting from 10 hit points, every additional 10 hit points sacrificed add another dice onto the attack's damage.

Caliborne may take 3 Legendary actions, recharging upon killing a creature, size medium or higher.

Summon Sirens. Caliborne may summon 4d8 Sirens of the Styx to its aid. 

Advanced regeneration. Caliborne may regenerate 2d4 tentacles. This ability does not work while Caliborne is prone.