Could the darkin be ascended+void?
The darkin were either created or summoned by the shurimans to fight the void. The shuriman's ascended were not wholely effective against the void based on the tales of their destruction in the story about the fall of icathia. Based on aatrox's interaction with void creatures I can't help but wonder if perhaps the darkin are some amalgamation of shuriman ascension and the void. This would help to explain their appetite for death as well as their hate for all targonian aspects (since the aspects seem to be at war agains the void (and in turn the watcher who control it)).
As a side note, we know that the void was originally brought to runeterra using rune magic. Ascension due to it's power to nearly ascend a being into godhood could also reasonably be explained by using rune magic and so combining them may not be an unreasonable feat.
that were touched by the void.