Shadow Priest Allister is a level 20 quest giver located at the Sepulcher in the Forsaken zone of Silverpine Forest.
Despite his low level, he must have been a powerful shadow priest while still alive; this is supported by the fact that he has written a book, [Holy Bologna: What the Light Won't Tell You], that eventually reached Dire Maul.
Shadow Priest Allister is a Forsaken leader and is nice enough, for a hideous undead monster, and is concerned that the Dalaran wizards are showing an increased interest in Silverpine.
He starts the following quests:
See List of Silverpine Forest NPCs.