I just love this game! Replayed it 3x, it's never the same. As usual, I am too lazy to review it myself so here's a review from Wiki:
Sacrifice is a real-time strategy (RTS)/action hybrid game that differs from most games in the RTS genre in that the player assumes control of a particular character on the field (the wizard) and plays the game from a third-person point of view centered on that character, and play is very direct, in that all creatures are created directly from spells cast by the player. In contrast, most real-time strategy games allow the player to see the battlefield and play the game from a top-down point of view, and force the player to create buildings which can later be used to manufacture units.
Sacrifice single player campaign consists of 10 sets of missions, one for each of five gods that the wizard can choose to work for. In addition to having different missions, each god also grants its own set of creatures for summoning and spells to be cast to the wizard. Not only does the player receive different spells from each god, but depending on which gods he chooses to support, the story changes drastically, creating several different endings. It is notable that once you serve a god, some other gods who sees him/her as a rival may not provide you any chance to serve him later, hence limited the god you can choose and avoid a "triumph combination" of creatures and spells.
The resources (a common element in RTS games) in Sacrifice are souls and mana.
Mana is used for casting any spell, summoning creatures, and is an unlimited, slowly generating resource. A manafountain is a neutral structure existing on the map and cannot be created throughout the game, but provides mana for any players nearby. To claim a neutral manafountain to be your own one, you can summon a Manalith on it, so it will provide mana only for you and your friendly creatures. Manahoars, a kind of summoned creatures which costs a soul, can draw energy from your manaliths and give you mana even though you are at a distance with your manaliths.
Souls are used for summoning creatures, and are limited; no new souls can be created during the game. Souls cannot be destroyed unless a unit falls off the map, in which case the soul or souls contained in that creature is forever destroyed. Several creature abilities, such as Consume Soul and Rend Soul, also destroy the soul but accompanied by other special effects.
Souls exist on the map in two forms: blue souls and red souls. Blue souls are either neutral souls that are placed on the map by the map makers waiting to be picked up by any wizards or souls that are released from the corpse of friendly creatures. These blue souls can be picked up directly by the wizard when he walks near it. Red souls are hostile souls that are released from the corpses of hostile creatures. Red souls cannot be picked up directly, and the wizard has to cast a spell Convert to summon a Sac doctor that will carry the corpse to its summoner's altar and carry out a ritual to purify the souls. During the transportation, the Sac doctor could be killed and the corpse release back to its former master.
The sole aim of this game to win, is to desecrate the enemy's altar. To do this, you have to approach your enemy's altar and cast a spell "Desecrate" with one of your creature as a sacrifice. A group of Sac doctors will be summoned and perform a ritual to desecrate the altar. During the process, the enemy wizard will suffer damage sent from the ethereal realm. The stronger the creature you have sacrificed, the more the damage is. If he is killed during the process, the desecration is done. If any of the Sac doctors are killed, the desecration will be interrupted and your sacrifice offering will be released free.
A god's power can reveal itself in any part of the world except other god's capitals, in which the Ultimate Altar of that god was set up to collect the faith of the god's people, which is the primary necessity of every god. Gods also establish a connection between themselves and their wizards through a wizard's altar. Thus wizards are the manifestation of gods to accomplish the god's will. Wizards can never be truly killed if the connection exists, and the god can resurrect the wizard somewhere else. But if a wizard is killed and he has no altar (or no connection with their god) at the time of death, they are dead forever and cannot come back.