Formerly, Onslaught was the leader of the Combaticons and formed the chest and head of the mighty warrior combiner, Bruticus, whose goal was basically to destroy. His original alt mode was a military style mobile infantry truck that could double as a small command base for the Combaticons.
Now Onslaught seems to be on more of a solo mission, choosing a new disguise as a S.W.A.T. styled vehicle that is used to break into buildings to disrupt the criminals activities and get through their defenses. It's doubtful that this is his goal, but this tough looking vehicle mode should cause the Autobots some discomfort on the battlefield.
At this point, we do not know if Onslaught will be joined by any of his former partner Combaticons, but we do know there will also be a Legends size version of him that is a repainted version of the Legends Cybertron Red Alert that is currently being found at some retail stores, but not officially due out until August.
You can view the still reasonably priced eBay auction here.
Thanks to Scaleface for the heads up!