Which Might and Magic universe are you referring to?

And another question for the experts. Did I understand correctly that the demons were finally locked away in Shio? Meaning they no longer escape from there during eclipses, and they are gone from Ashan? If so, in what year did this happen?

It all depends on Sareth's decision in 989 A.S.

In one version of the story, Sareth used the Skull of Shadows to seal the breach in the Sar-Ilam barrier, and the demons were forever trapped in Shio.

In the second version, Sareth destroyed the Skull of Shadows, and the Shio barrier was destroyed along with it.

And in the third version, Sareth decided to postpone the choice, and began carrying the skull with him all the time. And everything remained as it was.

In the game Heroes Kingdoms, the second version of events is considered. But the canonical version could be completely different.