"The lives of my troops are there for taking, to be used how I best see fit."Sacrifice is a fifth-level Death Magic spell in Heroes of Might and Magic IV that costs twelve spell points and takes the all of the hit points of one friendly stack of your choosing, and uses it to restore the hit points of another friendly stack, on a two-for-one basis.
When cast, Sacrifice will completely consume the stack you sacrifice, even if some of the hit points were not needed. Fortunately, like Resurrection, it will restore your creatures from death, even bringing back a destroyed stack.
This spell will always destroy the stack you sacrifice, even if not all of the hit points were needed. It must therefore be used with care; for while none would argue that restoring a dozen Devils is worth the lives of a few hundred imps, circumstance rarely provides such a favorable rate of exchange.
Fortunately, where circumstance fails, determination and magic can prevail: summoned, raised, charmed, and even purchased creatures can all make useful lambs for the sacrifice. In the late-game, one can free up a slot in their army by using the Undead Transformer on their imps, and having them march with the skeletons. Then fill the vacant slot with whatever might be purchased on the march. The terrible morale of your newfound 'friends' would be of little consequence, as they will just be padding for your real troops, with the aid of Sacrifice.