Familiars are minor demons that will serve you in exchange for soulstones. They bring with them minor spells that they cast on their master and his followers, and are useful in groups for this reason.
To summon a familiar, make sure you have a red or better soulstone in your inventory, then type:
summon demon familiar [demon type]
However, because they are very low level, they are fragile and are ill suited for combat. If injured, they will return to their home plane and consider their contract completed.
Advantages given by various demons:
A black, horned demon - hitpoints, damage resistance A winged red imp - move regen, attacks, mana An azure blue demon - elemental saves, elemental absorb An ebony scaled demon - hp regen, and saving spell, breath, poison An energetic green demon - mana regeneration and movement