One of my favorite top 5 games of all time. Sacrifice has aged gracefully and is still a lot of fun.
You control your wizard in real time from a 3rd person view and have a spell bar loaded with all of your offensive, defensive, creature summoning, building summoning, and other spells. Your summoned creatures are controlled directly by you and can be organized in formations. For instance, you put your melee troops in the skirmish formation so they lead the charge and then stick the ranged guys in a semi-circle behind them. You can have your units guard you as you run around the battle, or send them ahead and control the battle from a nearby hilltop while supporting with fireballs and heal spells.
The resources in the game are your life, mana, and souls. The souls work like a supply limit in games like Starcraft or Company of Heroes. You collect souls mostly from your enemy's dead creatures. Sometimes you can find them floating at a strategic point on the map or maybe even liberate a few from some local peasants. You life and mana are like most rpg/mmo games. You take damage from spells or creatures and you lose life. You cast a spell or are hit by certain creatures and you lose mana. You gain mana by casting manaliths on mana fountains -- think of it like capturing a point in Company of Heroes. The more mana fountains you tap with manaliths the faster your mana comes back, unless you get too far away from your manaliths... then it slows to a crawl again... so the lines of battle are drawn in a way. If a mage tries a direct assault on your shrine far from his own manaliths, he will run out of mana quickly and be left vulnerable. The shrines tether you to the world, so if they are destroyed you are likewise banished. If you die from loss of health, you can respawn after recharging at a mana source as long as your shrine is around.
The single player game has a well told story that changes based on the god's you chose to follow. It even has a collectible card game element to it. You can customize your spellbook to your liking by taking missions from different gods every time you play through. The god you pick grants you a unique spell and unit. So, there are many different spell and unit combinations... and game endings.
The multiplayer has AI opponents. Joining up with a friend against a team of 2 AI is quite enjoyable. There is no difficulty setting for the AI, but you can give them an advantage by adjusting their damage multiplier. Matches support up to 4 players with many different options to mess with -- like friendly fire, soul count, mage level, etc. The online service has been unusable for me, but hamachi works fine to connect with friends provided you do a direct ip connection to the host's hamachi ip (games won't show up in the local lan list, at least for me).
It is a great game which didn't get nearly enough attention when it was released. Full of innovation for its time, it is quickly forgotten that it is an 8 year old game. You will not be sorry you spent 6 bucks on this. With all the buggy, unfinished games often inflicted on pc gamers... 60 dollars might even seem a bargain for this treasure.