Arcane Well is a star within the Constellation The Apprentice in Elder Scrolls Online.
This was added in Update 6 with the Champion System. It gives you a 20% chance when you kill an enemy of opening an Arcane Well, which restores 795 Magicka to you and any allies within 2.5 meters of the enemy (at VR14). 120 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.