The Life Foundation
Leslie was a volunteer/mercenary for the sinister Life Foundation, an organization that was preparing for the M.A.D. (Mutually Assured Destruction) fallout of the Cold War and sought to provide a comfortable life for their wealthy clients after the impending nuclear holocaust. Leslie was picked from the organization's security force made up of cops, soldiers, and mercenaries to take part in an experiment. Venom went through a procedure where multiple 'offspring' were created from him - thus Agony, the product of a symbiote spawn and Leslie become one.
Leslie's lack of control over the alien inside her kept resulting in violence, so she and the other LF symbiotes kidnapped Eddie Brock in order to learn control over their powers. During Venom's time as Lethal Protector of San Francisco, Riot and her "siblings" were defeated by the combined efforts of Venom and Spider-Man. Leslie's symbiote was artificially aged to dust and his body was caught in a massive explosion.
It was later revealed that Agony and the other symbiotes had survived due to efforts by the Life Foundation. They broke Eddie Brock out of prison and kidnapped him in a last ditch effort to communicate with their alien "others." Unfortunately Eddie refused to aid them, due to Carnage's example that his "children" became psychopathic killers. Looking in a mirror at a hotel she and the others were staying, Leslie revealed her face and showed remorse for her actions as Agony. She was the first to be killed by Scream while offguard, the latter having snapped from her schizophrenia and her symbiote's influence, believing that the symbiotes and any/all who hosted them were evil. Leslie's death assisted Donna in misleading the others into thinking Eddie Brock was picking them off.
Leslie's symbiote (Agony) survived, and was imprisoned/experimented upon in The Vault along with its siblings (with the exception of Carnage and Scream.) The symbiotes eventually merged together, and bonded with a man known as Scott Washington to form Hybrid.
Agony was one of the five Life Foundation Symbiotes that were forcefully spawned from the Venom symbiote. When it was trapped after its host was killed, it merged with the other Life Foundation's symbiotes (except Scream), to create a new symbiote called Hybrid, which bonded with Scott Washington.
After Washington was killed by Eddie Brock, the new symbiote hunter, this symbiote was presumably found by the U.S. army and was defused into Lasher, Riot, Phage and Agony again. Each of the symbiotes were given to each member of the Mercury Team. Apparently, the defusion, or its near death, caused each symbiote to be weaker, unable to totally bond to its host.
The members of the Mercury Team helped Spider-Man, the new Venom, and Scorn battle Carnage, who was taking over the city of Doverton, Colorado.