The plot of Heroes of Might and Magic V.

I wouldn't call Arantir a villain. He fought against Sareth – the Dark Messiah. And it's not his fault that we play as Sareth. Sareth himself suspects at the beginning of the mission that the Dark Messiah is Arantir.

As for the expansions, I think they are very closely tied to the original game and to each other in terms of plot.