Written by Mr. Mouseburger 28th Feb 2010
Kindly submitted by KiraJenLove
By this point in the film everyone should have worked out that the lives of the Mystics and the Skeksis are inextricably linked (if not, then i have a) revealed a major spoiler and b) you are a pudding!), so that when one dies, its corresponding counterpart also dies.
Therefore when SkekTek (q.v.) falls into the firey chasm, he seals the fate of Urtih, the Alchemist among the Mystics who, while approaching the castle, spontaneously bursts into flames and disappears.