The Doctor and Rallon found the Celestial Toymaker together along with Millennia, another member of the Deca and Rallon's lover. The Toymaker took over Rallon's body and used it as his own, capturing Millennia as his servant.
During a confrontation with the Fifth Doctor, as Rallon's body began to decay, Rallon was able to expel the Toymaker by triggering all twelve of his regenerations at once, Rallon and Millennia passing on while his Watcher remained to keep the Toymaker in check in future. (PROSE: Divided Loyalties)
Behind the scenes Edit
Though Rallon to date has not been portrayed by any actors in any official Doctor Who media, it is reasonable to presume that, as the Toymaker is possessing his body during the events of his first appearance, that Michael Gough's appearance as the Toymaker can be considered to be considered as his own.
The notion that the Toymaker was possessing a Time Lord's body in his original appearance would seem to be a reference to the original authorial intent that the Toymaker was a member of the Doctor's own (then-yet-unnamed) species, which has later been overridden by the declaration that he was a Guardian.