If there is an imposter in Asgard, most likely Igron is behind the trickery. He learned from the best, allying himself with Loki. But even Loki didn't like this guy and made him a slave to the trolls.
Escaping from the mines of the trolls, Igron found Asgard empty (the Vrellnexians had enslaved them). When the Asgardians started to return, he used their own devotion and faith in the heroes of Asgard to create imposters of Odin, Thor, Fandral and others. Yet, these imposters were no match for the true heroes when they returned.
Mangog also employed Igron's illusions to adorn the likeness of Odin and the throne of Asgard. But when Mangog felt he no longer needed Igron's help, he dumped the sorcerer.