Mana is obtained by finding one of the mana founts scattered across the maps and building a manalith on it. Once you kill an enemy creature, you can summon a Sac Doctor, who will then drag that enemy's soul to your altar and sacrifice it. It's all pretty weird, but it works. While the campaign game is a lot of fun, it's ultimately a little disappointing because most missions tend to get settled in pretty predictable ways. This is a shame, because the creatures in Sacrifice have wildly different abilities and the spells are very original, and there are plenty of opportunities for tactical ingenuity.
Some creatures are good at melee, some at ranged combat, some can fly.
Almost all have powerful special abilities. High ground matters. You can easily group your creatures and arrange them in several formations, including line, circle, guard, and phalanx. I imagine that if I had to think a little, I probably might have used these abilities in ingenious ways. Which is why the multiplayer game is so good.