Sin was a homunculus and was the personification of the seven deadly sins. He is worse than Lust, Greed, Envy, Gluttony, Pride and Wrath all combined together. He is even worse than Dante (the creator of the homunculi in 2003 anime).
CreationPeter Evans was the brother of a state alchemist in the Amestrian military named Zachary Evans. One night when Peter was walking home after spending a long time at the pub he was attacked by a shadowy figure who whispered quietly, "blood", before stabbing Peter in the chest with an old kitchen knife. Peter was left mortally wounded on the ground sometime before his brother Zachary came to his aid and had lost large amounts of blood. When Zachary came to his aid Peter told him about his killer and how he had whispered, "blood" before killing him. Peter died shortly after telling his brother this but Zachary didn't want to say goodbye to his brother so he tried to bring his beloved brother back. Zachary attempted a human transmutation by sacrificing his left eye for his brother, but the transmutation had not resurrected his brother. It had instead created an evil homunculus who looked like his dead brother Peter, Zachary realized his mistake and fled from the homoncolous but the homunculus would pursue his creator. This homoncolous began to call himself "Sin" because it was its creators sin.
Sin tried to kill Zachary but was stopped by Edward and Alphonse. Edward and Alphonse had a big fight but lost and were about to be killed, but instead, Roy Mustang saved them and held off Sin for a while. However, Sin's strength overwhelmed Mustang and Sin killed Zachary. Mustang was hurt too bad to protect Zachary and ended up blaming Zachary's death on himself.
After Lust and Pride are killed, Sin is afraid that Edward and Alphonse will destroy him too so he begins training.
In the real world
Sin escapes to the real world where Edward and Alphonse are after the Conquer of Shamballa movie. Edward and Alphonse fight their best and defeat Sin (who was stronger than all homunculi combined) which somehow opens up a portal to their world. Edward and Alphonse escape back to Amestris and see Winry and Pinako again, also the portal closes so that Amestris and the real world will never connect again.