Increase stack size with sacrifice?

[jynki]

I head that you could increase the stack size of a unit with the spell sacrifice and thereby get high numbers of rare units. however i only seem to be able to regain lost units. i try to sacrifice high level units but when i try to use it on units with full health there´s just a question mark...

Am I doing something wrong or did they change the sacrifice spells? I dont use any of the latest 2 patches