Rejuvenation. If it has a phylactery, a destroyed lich gains a new body in 1d10 days, regaining all its hit points and becoming active again. The new body appears within 5 feet of the phylactery.

Legendary Resistance (3/Day). 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 two Fathomless Blade attacks or uses Spellcasting.

Fathomless Blade. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 7 (1d6 + 4) slashing damage, plus 9 (2d8) cold damage. If the target is a creature, it must succeed on a DC 17 Wisdom saving throw or be frightened of the lich for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Into the Deep (1/day). The lich magically opens a temporary conduits to watery destinations. 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: disguise self, mage hand, prestidigitation

3/day each: detect magic, fog cloud, shatter

2/day each: dispel magic, lightning bolt

1/day each: cloudkill, cone of cold, control water, whirlwind

Shape Up, Ye Dog (2/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.

The lich 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 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.