During the Autobot/Decepticon war, Outback and Kup crash-landed on an undiscovered planet that was covered with radioactive crystals. When the two of them attempted to use parts of their ship to build a transmitter, the crystal radiation overloaded the generator and it exploded, killing Outback. Kup unfortunately, wound up gradually being driven insane by the radiation, and began talking to the remains of Outback as if he were still alive, and also used his severed arm as a club.
Springer was later able to remotely hack into Outback's holo-emitter to send a message to Kup that they were coming to rescue him, but Kup's deteriorated mental state caused him to believe the hologram was a ghost threatening that "they were coming to get him", and that the people trying to rescue him were blood-thirsty ghouls.
Eventually Kup was able to be saved and taken away from the planet. Outback's remains were most likely taken as well.