i think the desciver is the smartest of the two fallen. But The Watcher is better. in mytyh TFL the watcher destroys D's army.
also the watcher has a greater experience of being evil.
1. he killed Mazzarin the most powerful sorcerer in the 4 ages. yes it is correct that it took him seven wawes of thralls to do it. but if anyone remember the the text:
he stumbled over the hill and saw the watcher then he knew the number of his days.that proves that Mazzarin couldn't defeat the watcher. and it is no execuse that he have fought agains seven wawes of thrall's. if Mazzarin had 7 of these alric magick exploding things. then he would easeliy defeat 7 wawes of thrall's
2.Connacyht could not kill him. connacht just trapped him in a moutain or something