The magical users dubbed "warlocks" are rare and shunned by almost all people. By pacting with a demon they are capable of harnessing Demonic energy (A tainted type of magica) and can even be taught demonic spells and abilities. In return they almost always have to give up something greater than they receive, or do actions that go against humanities best interests.

Although these type of magical users are often weak and pathetic creatures, occasionally they can become dangerously powerful with the right backing. 

If you suspect a wizard is actually a warlock, please contact any nearby authorities so they can be subjected to questioning.

Do NOT approach them yourself. Refrain from stabbing them in the back, even if they may deserve it.

-Adventurers Guide to the Continent