The magic system uses mana to cast spells, and mana regenerates over time. Regeneration is faster when close to your temple or near a mana fountain. Creatures called manahores may be summoned and if you are in proximity to them they help your mana regenerate as well. Mana is one resource in the game, and the other is souls. As a creature dies, its soul may be absorbed, allowing you to summon another creature.
Followers cost mana and a soul to create and there are only a limited number of souls available.
Absorbing the soul of one of your followers is as easy as walking over it. The souls of your enemies may be absorbed as well, but you must cast a spell that calls a creature that will trap the soul and bring it back to your temple to cleanse it. Only then may that soul be used by you. All of this forces some different types of strategies when engaging in a mission. The further you get from your temple or manaliths, the weaker you get due to mana regeneration, so sometimes it's wise to let the enemy come to you.