Vengeance is a star within the Constellation The Apprentice in Elder Scrolls Online.
This was added in Update 6 with the Champion System. It guarantees your next Spell will be a critical hit, after you block 3 Spells within 10 seconds of each other. 10 points in the Apprentice Constellation are required to unlock it. Once unlocked the player will acquire this bonus automatically and across all characters on their account.