[PvK]

My guess is the Doom Horrors leave your service as soon as they have something better to do (e.g., they go attack someone with a horror mark...).

So the only current way to mod that for a hero would be to give the hero the hard-coded unit number of one of the horrors... which of course would also mean he'd be the horror that would show up to kill horror-marked folks...

... if my guess is right.

PvK