A few seconds after you first pull Nortlav, a random group member will become disembodied, and Nortlav will take on the appearance of that player.
While the rest of the group continues to fight with Nortlav, the affected player, who is now in spirit form, must go to the back of the room and kill 3 waves of books that spawn on the book shelves in the center of the room.
If the books are not killed or any reach the rest of the group, they will summon a set of larger books on the rest of the party that have tons of hitpoints, and plenty of damage to boot, which will be very difficult to survive.
Once 3 waves are killed, kill the enlarged group member and the possessed party member will return to their body.
Be sure to give the possessed member enough time to kill the 3 waves of books before killing the enlarged encounter, or else the books will reach the named, and the adds will spawn.
Nortlav will randomly possess group members 3 times and die when the 3rd enlarged member is killed. NOTE: If you get possessed and "killed", your Stun buff might not come off. Simply cancel it from your Maintained Spells window. NOTE: If you are to slow the possession take place 4 times. Also after killing the 4th posessed member you still had to Kill Nortlav NOTE: If attempting to kill Nortlav with just yourself, or yourself and a mercenary, he will completely heal whenever he's attacked below 50% HP. This does not apply to effects already cast on him or passive damage caused by damage shields, though. A very high-level healer can eventually defeat him just by standing there.