"When Attumen or Midnight is at 25%, he will mount Midnight. This will heal Attumen to the same health as Midnight if he is lower, as well as reset all aggro. Attumen and Midnight must be in line of sight (LoS) to use this ability. If they are not in LoS, Attumen will despawn and the encounter will have to be reset."
So *lol* to lol989. :P
Except when your on the test realm and hortus comes in and one shots Attumen and crashes the instance