[Petwin]

Back to the original topic: the use of the spell Sacrifice. You can increase the number of a stack by sacrificing part of another stack. You can use it to grow your army for free (and there was much rejoicing..)

Basic troop gaining:
1) Sacrifice a lvl5 unit
2) Gain units for a stack <lvl5
3) Resurrect or Time Back the sacrificed unit.

But be careful: you can get more units in the 2nd stack than your leadership allows and then the stack will become out of control.

Additional things to consider:
1) With the spell Peacefulness the health of the sacrificed stack will be higher. In that case 1st level Sacrifice will not kill the unit.
2) When you resurrect damaged stacks there's a chance the resurrection will not work. Heal the stack before you resurrect.