Karthus was one of the members of Detheun's party (the scout party). Along with Reynard, Karthus died in the dungeon while they were trying to find out what happened to the trade caravans. The player's party will find him dead in the dungeon with the inability to resurrect him due to the burns inflicted while reincarnating.
The Soulharvester can likely create an enslaved form of Karthus to aid him and the betrayed Detheun into defeating the player's party.
After defeating both the Soulharvester and Detheun (or Possessor, if he was abandoned), Reynard and Karthus can now be resurrected with the appropriate rites.
"Enslaved soul of the fallen ranger, Karthus."