Spoiler warning: The following content contains unhidden spoilers for Act 3.
Metamage Persecutor is a hostile Countermeasure Aberration character in Baldur's Gate 3 who may appear during the final battle Atop the Netherbrain if the player sides with the Emperor. It will appear if a sorcerer is present in the party.
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.
You must be concentrating on another spell to cast this. If the target was concentrating, you heal as much as the damage that was dealt to it.
The spell you were concentrating on will end.
Range: 9 m / 30 ft
Notes
While the player character can only use Psionic Dominance once per long rest, the Metamage Persecutor is able to use it multiple times so long as it has a reaction to spend.