[Taqwus]

You can wish for one of any monster type which has a unique name. I haven't fiddled with seeing if there's any way to get a monster by unit number.

This includes pretender types, national heroes, et al. If you wished for a Patriarch, you'd get a unit of the same type as Father Bart (assuming no other Patriarch unit exists -- haven't played around with ambiguity, either), except that he'd be a unit and you'd have to Gift of Reason him to get his magic. If you wished for a Lord of the Night, well, you'd get a new Lord of the Night.

It's not perfect. If you wished for death, you might not die.