Warlocks are born from demons and humans, usually rape or deceit from shape-shifting Eidolon demons, disguised as humans or a human's loved one or an object of sexual desire.
Warlocks are immortal beings. At one point in their lives, they stop aging—which point exactly will depend on their warlock parent. At one point, however, after an unspecific but long duration of time, warlocks, like most immortal or long-lived beings such as Silent Brothers, can "fade" and "petrify"—when they grow bored or unsurprised by life and stop looking forward to their years ahead, when the weight of their long lives settle on them, they may begin to lose their minds, and/or stop feeling and living, in all other essences of the word except literally.
A universal trait among them are their warlock or demon marks, a part of their body that is strange or unique that identifies them as unearthly. These can be extra, inhuman body parts such as bat wings, goat feet, lizard tails, scaled hands, cat eyes, or differently-colored skin and hair. Additionally, warlocks are unable to have children. Warlock pregnancy always results in stillbirth as they are crossbreeds and are thus sterile.
An exception from these limitations are the very rare offspring of demons and unmarked Shadowhunters; without Marks to resist the demonic energy in a fetus, the pregnancy results in a living child—particularly, a dominantly Shadowhunter being with traces of warlock abilities. The only known case so far is Tessa Gray, who does not have a warlock mark and has given birth.Because of their half-demon, half-human nature, other Downworlders sometimes insultingly refer to them as "half-breeds."
- Yves Slate (High Warlock of London)
Magic: Warlocks can cast and have access to magic due to demon blood flowing in their veins. They are capable of casting spells and performing summoning rituals. These magical powers sometimes manifest themselves in physical form, such as flames dancing on one's fingertips or sparks floating around them.
Immortality: Warlocks are granted immortality, again by their demon heritage. They do not physically age. They'd only stop aging in a certain time and retain that appearance for as long as they love.
Accelerated healing: Warlocks heal faster than humans due to the demon blood they have.