Let me break it down.
A being like Eclipso would be detected by Master Order, and Lord Chaos, and even though these two may not be able to stop him alone bigger guns would soon show up to lock him safely away for all time (Order and Chaos have been in counsel with the LT and other high cosmics in the past).
Eclipso would shift the balance of order to become more chaotic, in which it is plausible as well as possible for LT to step in and do away with the upstart.
LT locked down the Starbrand, and the Infinity Gauntlet, so I'm pretty sure that he wouldn't allow the universe be turned into pure chaos by a being like Eclipso.
It's highly doubtful that any or all of the Marvel hero's would be able to stop Eclipso without PIS being involved.