During the M’arrillian Invasion, Khybon, brainwashed by Phelphor, under the command of the M’arrillian Chieftain Milla’iin, started building the enormous pipes needed to flood the Lava Pond with the waters of Everrain. An undertaking of such magnitude definitely couldn’t go unnoticed by the Creatures living there and, naturally, they fought back.
At some point after the confrontation between Zapetur and Milla’iin seen during the events of Newbie, be either by facing the M’arrillian himself or simply by getting caught in the cross-fire, Magmon ended up brainwashed as well. Eventually, with Milla’iin moving to the Marsh of Murk, Magmon came back to his senses, realizing the destruction caused to his home. Enraged, comes into play the Retaliator.
While a M’arrillian Minion, Magmon became even more powerful but, given his relatively impaired mental health, his fury degree made him an almost uncontrollable beast as well. But even if the trade-off for such power has rendered him much less courageous than before, with the needed comprehension to master a new Element essentially related to his volcanic nature as well, he awakened as an Earth Elementalist, expanding his range of Attacks as well as boosting the efficiency of the already trustable ones, like the Lavalanche.
And because the only way of breaking free of brainwashing is by terminating the link with Aa’une, Magmon would remain indefinitely susceptible to the M’arrillians’ mind-controlling abilities and his rage turned toward ta specific enemy could easily be switched against anyone unassociated with them in the presence of any other Chieftain.