Just as witches have existed since the dawn of Magick, so have Warlocks. Anytime there is ever something powerful in this world, there are those who go about getting it the right way and those who want a cheap and nasty shortcut.
The first warlock was said to be Lilith, first wife of Adam. She was said to have made her deal with the Devil himself.
Whether this is true or not, the warlocks of the modern age are each empowered by a different demon, and all demons are said to be children of the union between Lilith and the Prince of Darkness.
The warlock gets strength and magicks. The Demon feeds off the pain, death, and entropy they spread. Thus a compact is struck. One demon can only empower one warlock, should the demon find another they are enticed by, they would need to abandon their current warlock to do so.
As any cheat, this rarely bodes well for the warlock's long-term future, but this is rarely enough to deter them in the heat of the moment when a choice is offered between normalcy and power.
Every warlock deal is different. A demon will typically find those who are at rock bottom and offer them power in exchange for something they value: their soul, their morality, their dignity, regular sacrifices, etc. It will be something deeply meaningful to the prospective Warlock and something that will get progressively worse as the deal goes along.
Warlocks are generally human looking, at least at first. If they push their boundaries with magick too far or even just as a simple matter of sustaining a pact with their demon benefactor too long, they can develop inhuman traits as signs that their bodies are wearing thin. These can include eerily pale skin, bloodshot eyes, lesions, skin deterioration, and permanent damage from any destructive magic they perform.
Warlocks do not work in groups. Warlocks have an instinctive distrust of other warlocks as they are extremely possessive of the demons that empower them and vice versa. Warlocks will however work with other denizen groups except witches. Warlocks are one of the few with a (one-sided) grudge against that group, mostly born of a supernatural equivalent of professional jealousy.
Quasi Immortal. Technically immortal, immune to the effects of aging and sickness.
Warlocks can access the Source through an unnatural "backdoor" given by their demon benefactors, bypassing the work and art of witchcraft. Akin to someone being trained and entrusted with a job (witches) versus someone sneaking in the back and looting the place (warlocks).
Magicks. Their magicks are powerful but rudimentary. To this effect, the majority of warlock spells can be summed up by one word. "Push" "Destroy" "Block" "See" etc. It is not a skill set or a creative in any way; it is a loaded weapon handed to an angry child.
As previously mentioned as part of their bargain, Warlocks are given a small boon of physical empowerment. This makes them tougher, harder to kill, and stronger. They are stronger than a human should be capable of but still weaker than the superhuman strength of groups such as Werewolves and Vampires.
They have no need to dedicate the time and attention to witchcraft in order to do magick and are capable of magical acts that witches are forbidden as part of their craft. Unnatural and dangerous magick such as necromancy and psychic vampirism.
Unlike witches, who retain their own physical self and merely adopt magical skills, becoming a warlock involves some physical empowerment. When you first make a compact with the demon in question, your physical form is empowered, skin becoming more durable, and your physical strength increased.
Because of their magical shortcut, the power of magick may come easier to them than it does to witches, but their powers are far more limited. Not capable of the more versatile and detail-oriented craft of witches. Such perverse arts are also often detrimental to the warlock's health. The physical changes that becoming a warlock requires, compacted with having a demon benefactor, can also be quite harmful in the long term for the Warlock.