Thanatos is a dark reflection of Aquaman. He inhabited an "other dimensional realm" known as "Netherspace" that is a kind of reverse purgatory where beings known as the “Others” keep entities until they are filled with sufficient evil to be returned to the New Earth dimension.
He began as a being trapped in the Netherspace only coming out after luring victims to his mirror and stealing their identities. Regular citizens of Earth would be immobilized by him while he commit crime with their identity. When prompted for any sort of response the "zombified"citizens would only repeat "I am dead. Thanatos Killed me" over and over again.
One day Thanatos lured Aquaman to his mirror by sending him a message from "aqualad" asking to meet in the abandoned house the mirror resided in. Satisfied with the powerful identity Thanatos kept Aquamans form but was eventually banished back to the Netherspace.
After Aquababy died Mera went off on her and Thanatos was able to lure her into the Netherspace by convincing her that he was the real Aquaman. Nine months later she gave birth to A.J. who could easily be either mans son. In search of Mera, Aquaman finds his way into the Netherspace but is unable to leave as "the others" have not deemed him worthy. Aquaman and Thanatos engage in a final battle, in which Aquaman is the victor, but he refuses to kill Thanatos and the others see this as weakness and a lack of evil. They deem Aquaman unready to leave but during the battle Thanatos proved himself to be filled with sufficient evil. They allowed him to return to the New Earth Universe where he could continue to live as Aquaman.
Thanatos' returns to Poseidonis only to find it deserted and quickly grows bored. He uses a communicator to declare war on the surface world only to immediately be murdered by Major Disaster who was previously hired to kill the real Aquaman. Crushed between the closing walls of a trench he is presumed to be dead, leaving the surface thinking that the real Aquaman has declared war.