Riposte is a star within the Constellation The Atronach in Elder Scrolls Online.
This was added in Update 6 with the Champion System. While blocking, 15% chance to return you (Weapon Damage * 2) to the attacker when you block a melee attack. This can only happen once every 5 seconds. 30 points in the Atronach Constellation are required to unlock it. Once unlocked the player will acquire this bonus automatically and across all characters on their account.