Hehe, necromancy... can be totally cheaterific in MoM. If you go full Necro, I think you can start the game with the summon for Spectre (level 3 spell, if i am not mistaken?), which easily owns just about everything early on.
In college, I think I played this game to death, along with MoO.
You're probably thinking of Wraith. And yes, that's one way to win really quickly, especially since unless they have really high resistance, units killed by Wraiths are likely to rise up as undead to serve you. And remember that the undead version of normal (as in non-summoned) units have FREE UPKEEP!
My personal strategy is to start up an Artificer, Alchemy, Myrran guy with lots of Sorcery (blue) books, equip a single wizard-type hero with ultra-powerful artifcats (with Flight, Magic Immunity, and even the bugged Invisibility.) These take a looooong time to enchant, but then the guy (or girl) can proceed to pwn everything in sight. With a few levels under the hero's belt, even Sky-Drake filled sorcery nodes don't stand a chance (though they're still a real pain to clear.) Until you get the actual invisibility spell though, keep away from Nature nodes with lots of spiders - the artifact version won't stop them webbing the hero, and then he's wyrm food.