Critical Leech is a star within the Constellation The Lady in Elder Scrolls Online.
This was added in Update 6 with the Champion System. After Critically striking a target, restores (320) Health (at VR14). The Health restored scales with your character level. This can only occur once every 5 seconds. 75 points in the Lady Constellation are required to unlock it. Once unlocked the player will acquire this bonus automatically and across all characters on their account.