The Darkin and the void are both invaders from another dimension attempting to invade runeterra. But do the Darkin view the void as competition or another realm to conquer? Does the void see the Darkin and whatever dimension they come from as simply more food for them to eat? Maybe they dont mind eachother and will work together towards a goal if it benefits them.
The darkins are from another planet, not dimension.
im pretty sure they would be rivals but in the end i think the void would beat the darkin
I got the impression that the Darkin were magically powerful alien invaders that are, in some capacity, immortal. Like spirits that have built physical bodies for themselves or something.
Given that the Void devours living matter AND spiritual matter (souls) I think they would be just as skunked as anyone else if some voidmeat started nomming on them.
The Darkin view the void as enemies, and are terrified of them. The sole reason they even went to runeterra was to try to rile up and team up with the humans to destroy the void. However this didn't work for the humans feared and hated the darkin just because they were more powerful than them, and after the darkins taught the humans blood magic, in order to help defend themselves from the void, the humans betrayed the Darkin and Killed all but 5 generals. The 5 strongest darkin, were sealed away, those Darkin were. 
and 2 more unknown Darkin. There are theories but we don't know for sure who the last 2 are, however 1 was sealed in Shurima it is thought.
