The plot of Heroes of Might and Magic V.

Why isn't it necessary? To at least understand who Arantir is, you need to play Dark Messiah.

It's not necessary to understand. Arantir is Arantir, no matter what. I liked the developers' approach of showing him as the main villain in Dark Messiah (of course, only through Sareth's eyes) and as a necromancer with good intentions in the 2nd expansion. Tribes of the East only reveals some details from the plot of DoMM, and DoMM, in turn, continues the story of the heroes. But overall, the games are quite independent and not tied to each other in terms of plot, like the original and the expansions of Heroes are tied to each other.