So what exactly do i have to do to ressurect a particular hero ? I mean, i don't want to ressurect enemy heroes, but if one of my heroes and an enemy hero die in the same battle when i cast the spell it seems the enemy heroe is always ressurected first, which doesn't make any sense to begin with, and i'll have to keep re-casting until i get 'mine' back. Is there a way to have any control over who gets ressurected or not ?

I think that bit about compatible race and aligment is refering to the ability of re-hiring a hero once he gets ressurected. If the race and/or alignment aren't compatible with hiring, the ressurected hero just says goodbye and starts moving on his own on the map.

Still, doesn't seem much relevant to how which hero gets picked for ressurecting. If i kill an enemy hero and loose one of mine in the same battle why would the game assume i'd want to ressurect the enemy hero i just killed and not the hero i just lost ? It doesn't make any sense.

In any case, im leaning towards you being right about it being random as i see absolutely no way to control the spell.

Short answer: no, you can't pick because you can't pick. Re-cast, or possibly save and reload (don't know if that even works, probably not)

Long answer: You want to resurrect a hero. You do resurrect a hero.
That the hero wasn't one of yours is apparently not important to the spirit of magic, who is rather uninterested in mortal power games and wars, as can be seen with his disjointed and neutral quest blurbs.

I played around a bit with the spell and it seems it's random indeed, thanks. Well, thing is, i don't want to ressurect 'a' hero, i know exactly which hero i want to ressurect. You know, same way that when i cast ice shards i don't want them to hit 'a' unit, i know exactly which unit i want to hit ;)

In any case, thanks for the confirmation.

If you intend to ever use this spell, be sure to have units with Charm or Dominate in your party. You might want to give your leader Dominate at the start of the game (it can be a very powerful skill if you also get his DEF and RES to 10 as soon as possible). Or just hire 8 nymphs or ladies of pain (or spider queens or charlatans or Yaka avatars), depending on alignment and city availability