A major focus of the game is gaining the power to raise the dead, through rituals and spells - and then to use them as your personal army. In order to progress, you will need to explore dangerous crypts, dungeons, and wilderness areas - and for this, skeletons and zombies make ideal bodyguards.
As the game continues, you'll gain access to various other magical and undead allies/slaves, including the ghost of an ancient shaman, a reanimated flesh golem, a wight, and some daemons.