Staged Shadow (3/Day). When Zant's is reduced to 0 hit points, he uses his Warp legendary action and regains half of his hit points.

ACTIONS

Multiattack. Zant makes two attacks with his Twili scimitar. Alternatively, he makes four attacks with his shadow volley.

Twili Scimitar. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 8 (1d6 + 5) slashing damage.

Shadow Volley. Ranged Spell Attack: +10 to hit, range 90 ft., one target. Hit: 9 (1d8 + 5) necrotic damage.

Dark Vortex. Zant whirls around like a tornado with his scimitars. He can move up to 60 feet in any direction except up or down. Any creature he runs into must succeed on a DC 18 Dexterity saving throw or take 12 (2d6 + 5) slashing damage and be knocked prone.

Enter the description for how bonus actions work for your monster here.

Blink. After taking damage, Zant can magically teleport to an unoccupied space he can see within 30 feet of him.

Zant 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. Zant regains spent legendary actions at the start of his turn.

Dark Giant. Zant magically grows to become a Gargantuan creature. When transformed, Zant's AC becomes 22 (natural armor) and he can only make the following attack.
Slam. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 5) bludgeoning damage.
If Zant takes any damage in this form, he rapidly shrinks down to a Tiny creature and is unable to act for one round as he quickly hops around in pain, though his AC remains the same until he returns to normal size, which requires all of his movement.
Black Gate. Zant opens a portal that summons one shadow beast, four shadow deku babas, or eight shadow keese. He cannot summon more than one group at once.
Warp. Using illusory magic, Zant creates an interactable arena based out of a location from his opponents' memories. He will generally utilize the terrain in these illusions to his advantage, only being able to be struck once he exposes himself to attack.