Meh, I'd like to throw in my two cents here. :D
I think Marrowkais are balanced enough.
But about the ideas of a third spell:
1) I immediately had to think about necromancers. Isn't that what Marrowkais actually are? So what about a spell that raises an ... let's say an armored skeleton warrior every time it is used for back-up, attack, defense, and so on (Those should cost population though)?!
and 2) The bloody sacrifice :eek:
Hypothetically, this spell would kill the first unit you select, no matter how much health it has. Therefore, a second unit that gets selected gains a low/big amount of health from this sacrifice. So an almost dying juggerknight could be used to refill a giant's health in the best/worst case. It kinda resembles the Water Elemental sacrifice, except that the Marrowkai would heal only one specific unit by sacrificing any other not-enemy unit.
But those are just ideas. :)
