Hi there, this might sound odd... But I went and deleted my custom races (paying no heed to my currently custom heroes, as a matter of fact as kind of a test too 
).
I found that if you delete your custom factions, all your customs heroes will be assigned to "ALTAR"... Even the ones with death magic.. trying to start a game with an altar hero with death magic will crash the game.
http://imageshack.us/f/39/screenshotcopy.png/
Screenshot of altar hero with death magic .
Sicnerely
~ Kongdej
