After getting a hold of the allspark, a rookie, got the ability to reformat his body color color. Optimus Prime was believed to be dead, so a new (leader)prime had to be appointed. Archamis was rejected and got a case of megalomania. He went to the decepticons, although not really agreeing with there cause, created his own rouge group of Decepticons to fight against Starscream (as megatron was also believed to be dead) and renamed himself Nemesis Prime, keeping "prime" as a sort of trophy.
Nemesis is part of his own decepticon group, Vehicons Members of this faction are currently:
Other cybertronians are said to be Vehicons but have yet to be proven to be so.