on the level w/ Dr. Lindsay, after you get the "summon trapper spell, go back up to the floor w/ the wall w/ the hole in it the description will say something like "this hole is small enough for a small dog to fit through" summon a trapper, and go through the hole, there will be two horrors on either side of a bridge go acroos the bridge and step on a button w/ a purple rune marked on it, a door will open across from the wall w/ the hole, press A to kill the trapper, get the rune, (when you get the rune, a horror will break through the wall so be ready) after you kill the horror, go through the hole in the wall and under some cobwebs across the bridge will be themantorok rune codex