So there was a whole world of Darkin

They invaded Runeterra at some point. They created blood magic. The folks on Runeterra kicked their asses. This must have been some time AFTER the fall of Shurima since Varus had no memory of the place being anything other than a desolate wasteland.

So given that mortals wrecked their shit, not even Ascended mortals, these guys aren't on Nasus/Renekton level. Nor are they on Aspect or Void level. They sound like a bunch of opportunistic space invaders? Maybe? The whole thing seems kind of muddy. We've already got demons, the Void, and the harrowing posing major threats to life in general. Did the Darkin really need to be something other than another flavor of demons?