From time to time desperate, ambitious or curious individuals discover and use the occult knowledge necessary to make contact with one of the many powerful magical entities that wander Eisen. These entities may be spirits, gods, powerful monsters or other things that humanity has no knowledge of, or name for.
Once contact is made, he that cast the ritual beseeches the creature for power in exchange for their service. If the entity accepts their service then the two will sign a pact. From this moment onward and forever after, the human being is a warlock, and the creature is their Lord.
A Lord grants their warlock a small piece of their own power. The Lord will also serve as a tutor, teaching the warlock hidden knowledge and occult secrets of all kinds, including how to cast spells by invoking the hidden names of both the divine and the profane, as well as how to bargain with and enslave spirits and monsters.
Being instrument of a dangerous and unknown entity makes warlocks into pariahs by common folk who fear what their supernatural patron may demand. These fears are well-founded.
For this reason, warlocks typically practice their craft in secret, though there are some lands where they may act openly.