Wrath of Ro is located at ( 459, -16, -373 ) /waypoint 459, -16, -373 and is spawned approaching Krel-Ariak and letting him complete his chanting. Once the Wrath is dead, he summons several, ever increasing waves of pyrelords.
Move to the staging area at ( 385, 18, -237 ) /waypoint 385, 18, -237 and kill the waves of Doomfire guardians to spawn Vengeance of Ro. Krel-Ariak appears at the side of the platform, and joins in when you attack Vengeance, but he dies easily. Beware the knockback in this fight which can punt you into the lava.
A portal to return to Myrist, the Great Library spawns after killing the last named and is located at ( 428, 20, -262 ) /waypoint 427.97, 19.66, -261.89