Ok, here's an Idea(tm) i've been considering recently. Maybe the Fallen Lords didn't have complete control over the Shades as they thought they did. MB the Shades were able to exercise some individual control (but were still intent on destroying life). Phelot may not have belonged to any specific Fallen Lord, he may have been acting indepentantly, knowing his actions would cause Shiver and Big D. to fight, and get rid of a bunch of myrks at the same time.
Just an idea...MB the fallen lords underestimated the powers they were resurrecting.