[Nick_K]

If you're going to have pretenders appearing as physical units in the world, and you're going to have physically-average pretenders such as archmages in the game then you have to accept the possibility of regular death. You can get the same result if you capture the enemies home province and simply kill them as soon as they're called back, for example. I'm sure you can also regularly kill a pretender with spells or assassination.
There are a number of ways which a pretender can protect itself which have been mentioned here. Primarily by casting returning or by not getting horror marked in the first place. I've played around a half-dozen SP games and the only horror marks I've ever received have bee nfrom items. It is something that the player has a lot of control over.