You have learned to harvest the souls of creatures you defeat in combat. These souls can be used in creating a spell to summon that creature to protect you.
To summon a creature, first craft a Crystal Phylactery. During spell creation, you can throw in any filled phylactery when prompted by the lectern. This step is what determines what your spell will summon.
Then, summon creatures by using the spell normally. The stronger the creature, the more mana this requires!
Tamable creatures such as wolves and cats are automatically tamed to their owner upon summoning. Other creatures are not, however they will still fight for you like a wolf would: attacking what you attack, and defending you from attackers.
Be aware that they will still have their usual weaknesses. For example, summoned zombies will still burn in sunlight, so you'll have to take precautions for that.
Summoned creatures drop no loot other than what was given to them (such as armor), as well as no experience. However, they can be interacted with normally otherwise, such as breeding, milking cows, or riding horses.