Many Illthid Arcanists will appear during several combat encounters at the High Hall and Atop the Netherbrain.
Illithid Arcanists have 2 variants: one with Fire Shield: Chill and Fire resistance, and another with Fire Shield: Warm and Cold resistance. Their version of Magic Missile deals more damage, fires more missiles, and has a significantly longer range. This makes them dangerous if not dealt with quickly. If pushed or thrown off the top of the Netherbrain during the final battle, Illithid Arcanists will simply teleport back to their former position.
If it is higher level than the spell slot you used to Counterspell, you must make a check using your spellcasting ability[See Notes] to prevent it. The check's difficulty is equal to 10 plus the level of the spell you are trying to counter.
Dominate a nearby humanoid. Allies are unaffected.
Each time the dominated creature takes damage, it may repeat the Saving throw against your domination.
When you are about to be hit by an enemy, use your Reaction to increase your Armour Class by 5. You also take no damage from Magic Missile. These effects last until the start of your next turn.