The two resources of the game are souls and mana. Your wizard need only touch these blue souls to return them to his collection, where they can be used to summon another creature. The death of an enemy creature produces a red soul, which cannot simply be picked up. This summons a little helper-creature who then proceeds to work at capturing the heathen soul. Once captured, he carries the creature's body back to your alter, or main base, and then proceeds to sacrifice it, the end result of which is the depositing of the enemy creature's souls in your cosmic bank account.
Souls are the key to the game.
Even if all your creatures die in a battle, if you can pick up their souls and escape, nothing has truly been lost. Melee fighters can stomp those wimpy ranged fighters, but are vulnerable to flying troops, but flying units are sitting ducks for ground-based ranged units. This is a simple system, but it provides much of the strategy of Sacrifice. One of the better tactics of the game is to save some souls before a battle, and then observe what types of units your enemy is using. Although use of summoned troops is the primary form of combat in Sacrifice, the your wizard's spells also affect gameplay.