Chronurgy Wizard

Special Equipment. The Chronurgy wizard uses a +3 Wand of the war mage. This gives +3 to attack rolls for spells. He also wears a ring of protection. The Chronurgy Wizard wields a blast scepter. It can be used as an arcane focus. Whoever is attuned to the blast scepter gains resistance to fire and lighting damage and can, as an action, use it to cast thunderwave as a 4th-level spell (save DC 16) without expending a spell slot. The chronurgy wizard also wears a horned ring which allows him to ignore under mountains magical restrictions. Finally, The chronurgy wizard wears a robe of eyes that lets him see in all directions, gives him a darvision of 120 feet, grants advantage on Wisdom (Perception) checks that rely on sight, and allows him to see invisible creatures and objects, as well as into the Ethereal plane, out to a range of 120 feet. He also wears a necklace (see lair description). 

Arcane Recovery (1/day). When the Chronurgy wizard finishes a short rest, he recovers all his spell slots of 5th level or lower. 

Legendary Resistance (1/day). If the Chronurgy wizard fails a saving throw he can choose to succeed instead.

Rejuvenation. If the Chronurgy wizard dies without having a (non-legendary) action in combat he revives 1d10 days later with all his hit points and any missing limbs restored. His new body appears in a random safe place in Undermountain. If he does combat again with any of the same enemies within the next century, he gets advantage on initiative and gets a +8 bonus to initiative. 

Chronurgy Wizardry. The Chronurgy wizard learned spellcasting at the school of Chronurgy. This means that every round, on initiative count 20, every enemy of the Chronurgy wizard must declare everything that they will do on their next turn. They must do what they declared if they can. If they are under a condition preventing them from doing their declared initiative then they must try to stop that condition. If they are being prevented by doing declared action in any way, the enemy must try to do what they declared. 

Spellcasting. The Chronurgy Wizard is a level 20 spellcaster. Its spellcasting ability is Intelligence (spell save DC 25, +17 to hit with spell attacks). The Chronurgy Wizard has the following Wizard spells prepared:

Cantrips (at will): Light, Mage hand, Fire Bolt, dancing lights, prestidigitation

1st level (4 slots): Mage armor, magic missile, shield, silent image

2nd level (3 slots): arcane lock, cloud of daggers, darkvison, knock

3rd level (3 slots): counterspell, erupting earth, fireball

4th level (3 slots): Blight, Dimension Door, Black Tentactles

5th level (3 slots): Bigby's Hand, Cloudkill, Wall of force

6th level (2 slots): Chain Lightning, Circle of Death, Investiture of flame

7th level (2 slots): Delayed Blast Fireball, Finger of death, Whirlwind

8th level (1 slot): maze, mind blank

9th level (2 slots): meteor swarm, wish

Blast Scepter. The chronurgy wizard uses his blast scepter to cast thunderwave as a 4th level spell (save DC 16). 

The Chronurgy wizard can take 3 legendary actions, choosing from the actions below. Only one legendary action option can be used at a time and only at the end of another creature's turn. The Chronurgy Wizard regains all spent legendary actions at the end of any creature's turn. 

Intervene (Costs 3 Actions). With the knowledge of what the Chronurgy wizard learned on initiative count 20, he can use this action to intervene and take an action to prevent the intended turn, one count, before it happens. For example, the first round happens without the Chronrugy Wizard having any knowledge of how it will play out. Then, in the second round, the Chronurgy Wizard can use this intervention to try to stop what the creature wants to do, before the creature does it, with the knowledge of what he learned on initiative count 20. 

Heal (Costs 1 action). The Chronurgy Wizard heals 2d8+5 hitpoints