The fel reaver's cannons continously alternate between [Annihilation]
Upon reaching 65% and 25% health, the fel reaver activates its [Apocalypse Drive]
exposing its weapon platforms. Unless one of these weapons is destroyed before the cast can complete, the party will be eradicated.
On Mythic difficulty, the cannons can no longer be destroyed, but instead go haywire, granting the cannons additional effects.