Kevin Tremain was a mutant captured and studied by the Mandarin. On a secret mission, the Six Pack attacked the secret base Tremain was held in. Tremain was mortally injured; Cable tried to save his life, first by using his telekinesis to keep Tremain's body together, and finally by giving him a blood transfusion. Although it seems he survived this trauma, Cable seemed to think Tremain had later died.
Years later, Tremain resurfaced as Post, the lowest of Onslaught's emissaries. Onslaught forced Gateway to teleport Cyclops, Storm, Iceman, and Wolverine to where Post could "test" them. The four managed to defeat Post and were transported back to the Mansion. Later, Post attacked Cable, who instantly recognized his identity as Tremain. Post defeated Cable and left him for the Hulk to finish off. Post was later charged with ensuring the Sentinels would launch, he battled X-Factor with them and succeeded protecting the Sentinels. He and his master Onslaught were eventually defeated by the heroes.