Brian the lich

Legendary Resistance (3/Day). If Brian fails a saving throw, he can choose to succeed instead.

Duality. Due to the nature of the possession and control between the staff and the necromancer the entity of Brian the lich can hold concentration on 2 spells at once. It's also this nature that gives Brian the ability to attack and cast or cast 2 spells as an action. 

Magic Resistance. Brian has advantage on saving throws against spells and other magical effects.

Regeneration. Brian regains 25 hit points at the start of his turn. If he takes fire or radiant damage, this trait doesn’t function at the start of his next turn. He dies only if he starts its turn with 0 hit points and doesn’t regenerate.

Spellcasting. Brian is a 14th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 20, +12 to hit with spell attacks). Brian has the following wizard spells prepared:

Cantrips (at will): acid splash, chill touch, mage hand, poison spray, prestidigitation

1st level (8 slots): magic missile, ray of sickness, thunderwave

2nd level (6 slots): Melf’s acid arrow, ray of enfeeblement, rime's binding ice, web

3rd level (6 slots): animate dead, lightning bolt, vampiric touch

4th level (6 slots): blight, dimension door, ice storm

5th level (4 slots): cloudkill, wall of force

6th level (2 slot): circle of death, chain lightning

7th level (2 slot): finger of death, forcecage

Turn Resistance. Brian has advantage on saving throws against any effect that turns undead.

Undead Fortitude. If damage reduces Brian to 0 hit points, he 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, he drops to 1 hit point instead.

Multiattack. Brian makes two attacks: one with his Noxious Touch and one with his Staff of the forgotten one. He can cast a spell with a casting time of 1 action in place of one or both of these attacks.

Noxious Touch. Melee Spell Attack: +12 to hit, reach 5 ft., one creature. Hit: 28 (8d6) poison damage, and the target must succeed on a DC 20 Constitution saving throw or be poisoned 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.

Staff of the Brian. Melee Weapon Attack: +12 to hit, reach 5 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage plus 13 (3d8) poison damage and 13 (3d8) necrotic damage.

Brian can take 5 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. Brian regains spent legendary actions at the start of his turn.

Cantrip. Brian casts one of his cantrips.

Noxious Touch (Costs 2 Actions). Brian uses Noxious Touch.

Disrupt Life (Costs 3 Actions). Each creature within 30 feet of Brian must make a DC 20 Constitution saving throw, taking 35 (10d6) necrotic damage on a failed save, or half as much damage on a successful one.

Invoke curse (Cost 2 Actions). The target must succeed on a Constitution saving throw DC20 or be cursed. While cursed in this way, the target can’t regain hit points and has vulnerability to necrotic damage. A greater restoration , remove curse or similar spell ends the curse on the target.