Once again The Allspark
comes through with a translation of the story that comes with the Transformers United EX toys. This time they have shared a translation of the third story in the United EX plotline, Prelude to the Siege. This story chapter introduces Combat Master into the battle between Chopper Master and Roller Master and establishes how he comes to acquire his own set of Power Core Combiner drones. The story also brings in a new big bad behind the Decepticons known merely as "High Regent"; given the timeline placement of this series in the late Japanese Generation 1 where the Decepticons were usually the thrall of some extra-dimensional entity or other (see also: Violenjaeger, Dark Nova) this is not unprecedented - but given the way that so far United EX is weaving in many plot threads from across the whole spectrum of Transformers lore, don't discount the possibility that this might be someone we've met before!
The story and bio translation establish a few other details about Transformers United EX, namely that the Transformers United EX story is happening in parallel with Operation Combination (and that the story of Operation Combination was in fact merely a distraction for the events of this story!) Moreso, the story also establishes that Combat Master is Onslaught
reborn - in the story illustration the Bruticus redeco, Battle Gaia, can be seen and is referenced in the plot as being "based on Combat Master's data". The bio backs this up with a very Onslaught bio of being a robot who prefers to let his subordinates do the fighting, and also with references to how he has been active since the early days of the war. Check out the full translation over on the Allspark