Blood Frenzy. The shark has advantage on melee attack rolls against any creature that doesn't have all its hit points.

Pack Tactics. The Zombie Shark has advantage on an attack roll if at least one of the Zombie Shark's allies is within 5 feet of the creature and the ally isn't incapacitated.

Undead Fortitude. If damage reduces the zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Water Breathing. The shark can breathe only underwater.

Necrotic Infection. If the Zombie Shark's bite successfully hits it may as a bonus action release a necrotic substance from its gums. The target must make a DC 14 Constitution Saving throw. On a failed save the target takes 2d6 necrotic damage and their hit point maximum is reduced by the amount of necrotic damage taken this turn.

Swift Swim. If the Zombie Shark is hit by an attack it may use its reaction to move up to half its movement speed.

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

Rotting Flesh. If the Zombie Shark takes slashing or piercing damage it releases a cloud of toxic mucus. The area within a 15ft radius of the Zombie Shark becomes heavily obscured. Every creature within 15ft of the Zombie Shark must make a DC 18 Constitution saving throw. On a failed saving throw the target becomes paralyzed, they may reroll the saving throw at the end of their next turn. If you succeed you become immune to the effect.

Bite. Melee Weapon Attack: +9 to hit, reach 5 ft., one target. Hit: 22 (3d10 + 6) piercing damage. plus 9 (2d8) Necrotic damage.