Either the Aphillion or the "Nothing". Chaos king clearly has the power to corrupt and shackle minds perhaps the only thing that could stand a chance against him is the lack of mind itself? Honestly we don't even know what the Aphillion and the Nothing are so I'm just going with the theory that Nothing is literally just an endless growing void that turns everything it touches into nothingness (pun intended) Kinda like the patterned screamers form the SCP universe almost