anti-Necropolis faction

My IMHO. The most formidable opponent for the Necropolis is Haven. Wing and Ossir leave little chance in the late game. Wing eliminates a significant portion of the army in the first turn (often some units move twice before the Necropolis units even get to move) + mass haste or punishing. Ossir - two shots from archers, and the vampires are gone. Darkness isn't very scary, as there's a lot of resistance + antimagic + cleansing.
A fast game as Necropolis, especially if it's Casper, gives Haven a much better chance.