Beholder Mage

Absorption Cone. The Beholder Mage has the Magic Resistance trait (advantage on saving throws versus spells and magical effects) while this cone is active. When it succeeds on a saving throw to which Magic Resistance applied, this cone becomes inactive. The beholder may activate it at the start of each turn. If it does, it gains temporary hit points equal to five times the level of the last spell or effect it resisted (20 if it had no level), and any expended spell stalks (see below) regain one prepared spell of the beholder's choice.

Mage Hands (four short swords). Melee Weapon Attack: +2 to hit, reach 35 ft., one to four targets. Hit: 5 (1d6+2) piercing damage each.

Spell Stalks. The beholder casts three spells it chooses, from three spell stalks of random levels (roll 1d8, rerolling duplicates and expended stalks).
All DCs are 16 and all spell attacks are at +8:

1. 1st Level Stalk. The beholder may cast Grease, Magic Missile, Alarm, or Detect Magic in an up to 8th level slot.
2. 2nd Level Stalk. The beholder may cast Scorching Ray, See Invisibility, or Invisibility in an up to 8th level slot.
3. 3rd Level Stalk. The beholder may cast Dispel Magic, Fear, or Haste in an up to 8th level slot.
4. 4th Level Stalk. The beholder may cast Stoneskin, Dimension Door, or Arcane Eye in an up to 8th level slot.
5. 5th Level Stalk. The beholder may cast Hold Monster, Telekinesis, or Wall of Force in an up to 8th level slot.
6. 6th Level Stalk. The beholder may cast Eyebite or Disintegrate in an up to 8th level slot.
7. 7th Level Stalk. The beholder may cast Finger of Death or Prismatic Spray in an up to 8th level slot.
8. 8th Level Stalk. The beholder may cast Feeblemind or Mind Blank in an up to 8th level slot.

The Beholder Mage can take 3 legendary actions, using the Spell Stalk option below. It can take only one legendary action at a time and only at the end of another creature's turn. The Beholder Mage regains spent legendary actions at the start of its turn.

Spell Stalk. The beholder casts one spell it chooses, from a random 1st-4th level spell stalk (roll 1d4, reroll if expended).