Click the second rune foundation at ( -91, -39, -268 ) Copy/waypoint -90.75, -38.95, -268.20.
Click the third rune foundation at ( 90, -38, -269 ) Copy/waypoint 89.63, -38.45, -269.17.
Run past the final rune foundation and around the bend in the corridor beyond to ( 262, -39, -264 ) Copy/waypoint 262.25, -38.73, -264.39.
Slay Izzak Sira. If you can't simply burn him down, then lead Izzak Sira to the each of the other three rune foundations. There will be a different colored circle on the ground next to each foundation. Pass him through the circles until you receive the message that he can take damage now, then kill him.