Essentially, there is a disparity of sorts between the notes and sources the RPG is based on and the numbers involved. This is really not a problem, as it is one of Naar's explicit powers to be able to spawn and even ressurrect minions not directly slain by divinities or agents of divinities other than himself.
Thus, there is no real paradox as Naar could simply (and as far as the author is concerned did) bring back Darklords once other Darklords slew them. Odds are, this happened ~constantly~ in the early years of the Darklands as the Darklords jockeyed for position. Odds are each death took its toll on their powers to the point where the Darklords agreed silently not to kill each other any more... unless it suited their needs.
After all, if one considers the non-chalant way Gnaag takes the death of Taktaal in the throne room of Helgedad, it could easily be seen that Gnaag assumed his 'brother' would return soon. If they were really deathless beings from which obvilion was inescapable, he would probably have taken the annihilation of a 'loyal' Darklord a LOT harder.
Just a thought,
-August