Brine Lich

Regeneration. While onboard its ship, the lich regains 15 hit points at the start of its turn. If the lich takes radiant damage, this trait doesn’t function at the start of the lich’s next turn. The lich dies only if it starts its turn with 0 hit points and doesn’t regenerate.

Rejuvenation. If the lich dies and its ship still exists, it comes back to life with all its hit points in 1d10 days in the captain's quarters.

Legendary Resistance (3/Day, or 4/Day in Lair). If the lich fails a saving throw, it can choose to succeed instead.

Sea Legs. The lich has advantage on ability checks and saving throws to resist being knocked prone.

Undead Nature. The lich doesn’t need air, food, drink, or sleep.

Multiattack. The lich makes three Fathomless Blade attacks.

Fathomless Blade. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) slashing damage, plus 13 (3d8) cold damage.

Ocean's Vengeance. The lich points its finger at a creature it can see within 120 feet of itself, calling the wrath of the sea upon it. The target must succeed on a DC 17 Constitution saving throw or take 26 (4d10+4) cold damage and fall prone.

Into the Deep (1/day). The lich opens a temporary conduit to a watery destination. As an action, the lich teleports itself. The lich, along with any equipment it is wearing or carrying, vanishes and then reappears up to 1 mile away in a body of water (pond sized or larger).

Spellcasting. The lich casts one of the following spells, using Intelligence as the spellcasting ability (spell save DC 17):

At will: fog cloud, mage hand, prestidigitation

3/day each: dispel magic, lightning bolt, thunder step

1/day each: cone of cold, whirlwind

Shape Up, Ye Dog (1/day). Whenever a friendly creature within 30 feet of the lich that can hear it misses with an attack, the lich can yell perilous threats to allow that creature to reroll the attack roll.

Parry. The lich adds 5 to its AC against one melee attack that would hit it. To do so, the lich must see the attacker and be wielding a melee weapon.

The lich can take 3 legendary actions or 4 legendary actions in Lair, 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 lich regains spent legendary actions at the start of its turn.

Attack. The lich makes an attack.

Frightening Gaze (Costs 2 Actions). The lich fixes its gaze on one creature it can see within 10 feet of it. The target must succeed on a DC 17 Wisdom saving throw against this magic or become frightened for 1 minute. The frightened target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. If a target’s saving throw is successful or the effect ends for it, the target is immune to the lich’s gaze for the next 24 hours.

Sap Life (Costs 2 Actions). The lich targets one creature it can see within 30 feet of it. The target must succeed on a DC 17 Constitution saving throw or take 11 (2d10) necrotic damage. The lich regains a number of hit points equal to the amount of damage that the creature takes.