Transformers: Rise of the Dark Spark
Chronologically, Onslaught's first appearance is in "Rise of the Dark Spark". He and the rest of the Combaticons were tasked with escorting Shockwave to Kaon. After an altercation in which Blast Off and Swindle protected a pinned-down Shockwave, Onslaught ordered the Combaticons to merge into Bruticus in order to clear a path through the remaining Autobots. As Bruticus was making his way towards the final gate, he was attacked by Cliffjumper, whom he incapacitated and then took prisoner.
Transformers: Fall of Cybertron
Onslaught would not be seen again until "Fall of Cybertron", in which he and the Combaticons are ordered by Starscream to hijack an Autobot Energon transport. While the rest of the Combaticons would personally attack a bridge to change the transport's course, Onslaught assumed command of their dropship and coordinated from a distance. When Starscream opted to attack the transport despite the Combaticons' warnings of heavy anti-air resistance, Onslaught ordered Swindle to hastily disable the transport's guns, even flying the dropship close enough for Swindle to use its weapons to finish off the last gun. Upon destroying the last gun, Onslaught ordered the Combaticons to form Bruticus in order to take the bridge while the Autobots are still collecting their bearings. While Bruticus did succeed in taking control of the transport, he accidentally knocked out a vital engine, forcing the transport to crash.
Due to Starscream's incompetence, he blamed the crash and loss of resources on the Combaticons, ordering their arrest despite their success in light of his failures. After Soundwave managed to revive Megatron, the two confronted Starscream's forces and usurped Starscream's command, freeing the Combaticons. Megatron then led a raid on an Autobot research facility to retrieve Trypticon's corpse, with Bruticus breaking down the main door.
Bruticus would later participate in the Decepticons' battle for control of the Ark. He and the Combaticons were ordered to destroy the Ark's fuel tanks, but he was thwarted by Jazz and Jetfire. Due to Jetfire's air strike, Bruticus was knocked off the Ark he and the rest of the Combaticons presumably fell back to Cybertron. It's unknown if any of the Combaticons survived, but the lack of mention in later arcs of the Aligned Continuity suggests that they either died from re-entry into Cybertron's atmosphere or impact with the planet.