Yiln Darksly is located in the Overthere fort, behind the boats you gate into with OT hammer, at pos 2689, pos 3241 location. Hope this makes a bunch of you happy.
Oh and on a side note, this guy is not on Overthere Fort faction, cause im dubious to them and I was apprehensively to Yiln Darksly. No OT fort faction people near him so probally can just invis to him for turn in, but I would con him from a far to be sure your not kos first.
... I would say that at a library or guild somewhere, some caster type person is looking for their shipment of missing books. Maybe there's a quest NPC at one of the caster guilds in a city somewhere, or one of the more widely known libraries, like Thurgadin or Erudin.