Aoskar

Ethereal Jaunt. As a bonus action, Aoskar can magically shift from the Material Plane to the Ethereal Plane, or vice versa.

Keyhole Vision. Aoskar is able to see through any portal, door, or such and view the landscape on the other side.

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

Spellcasting.  Aoskar is an 18th-level spellcaster. His spellcasting ability is Intelligence (spell save DC 17, +9 to hit with spell attacks). Aoskar has the following wizard spells prepared:

Cantrips (at will): light, mage hand, prestidigitation

1st level (4 slots): detect magic, identify, mage armor*

2nd level (3 slots): mirror image, misty step

3rd level (3 slots): counterspell, fly,

4th level (3 slots): dimension door

5th level (3 slots): scrying

6th level (1 slot): tree stride

7th level (1 slot): teleport

8th level (1 slot): maze

9th level (1 slot): time stop

*Aoskar casts these spells on itself before combat.

Dagger. Melee Weapon Attack: +4 to hit, reach 5 ft., one creature. Hit: 7 (1d10 + 2) piercing damage, and the target must make a DC 11 Constitution saving throw, taking 18 (4d8) planar damage on a failed save, or half as much damage on a successful one. If the planar damage reduces the target to 0 hit points, the target is stable but paralyzed for 1 hour