Forsaken are beings who were once mundanes who have gone insane upon receiving powerful runes they are unable to handle. They cannot be returned to humans and have no knowledge of their previous existence. They come in different forms.
On people who do not have Shadowhunter blood, a single Mark is likely only to cause a burning pain where it has been inscribed on the skin, but a number of Marks, particularly powerful ones, will drive a mundane to agonizing pain and mindless, insane rage. The Mortal Cup, or inherited Shadowhunter blood, steels the body against the overwhelming strength of angelic power that flows through the Marks, but an unprepared mortal will become Forsaken and eventually die.
Forsaken are suggestible and hold some affinity and loyalty towards the one who Marked them, making them mindless servants who fall under their command. While very occasionally used in slave labor, their unending rage and pain make them useful only for committing violence. While they can understand other simple instructions, they are incapable of speech and self-defense, and they are unable to build or construct anything, wield weapons, or implement tactics. They are weak and unintelligent creatures, making them easy to neutralize threats and terrible soldiers.
Also, unless ordered to do so, they do not eat or sleep, and they ignore their injuries and wounds. As a result, they are short-lived creatures, and mostly die through starvation, exhaustion, or infection.
As early as the mid-1200s, the Silent Brothers have apparently begun searching for a cure for the Forsaken. The term Forsaken was used in the first written version of the Covenant Law by the first Consul, Edward the Good and Ready, where they were condemned as illegal and blasphemous.
Valentine Morgenstern once used the mindless Forsaken as warriors for his army, letting them protect his hideout at Renwick's. He intended to use the Mortal Cup to make them into an army of Shadowhunters, but Luke's wolf pack killed most of them, not leaving enough for an army.