Wrath is a homunculus that was created by the failed human transmutation of Izumi Curtis' child. Izumi seeing that what came back is not her child, she gave him to the gate only to regret it greatly. Wrath manage to free himself after along time.
When Izumi Curtis' child died she tried to bring him back using Alchemy and the remain of her child body, at the cost of losing her capability to have another child. Izumi believed that she succeed, but was horrified to see what the body of her baby turned into. Understanding that what has returned was not her child and so she gave him to the gate only to regret it greatly after hearing the child cries. Wrath spent along time in the absolute darkness of the gate wanting to get out while his body was slowly growing along with his hate to the mother who condiment him to this fate. When the Elrics brother tried to bring back their from the dead, Wrath took the leg and arm of Edward, which gave him the ability to use Alchemy and after along time he finaly manage to open the gate and free himself. Emerging at the same spot Izumi tried to bring her baby back to life, but lost his memory of who he is and what brought him there.