Armothy became a part of Morty when Rick Sanchez transferred Armothy's physique and muscle memory into Morty's arm to distract other inhabitants by fighting in the Blood Dome (or the Thunder Dome), whilst attempting to steal a large deposit of Isotope-322. The arm moved and "thought" independently from the rest of Morty. Though Morty was at first horrified, he soon used Armothy to vent his frustration toward his father Jerry for agreeing to divorce Beth. Eventually, Armothy found the leader of the invaders who ordered the attack on his village. Morty, realizing that once Armothy's revenge was carried out he would disappear, tried to get Armothy to delay revenge so Morty could continue to vent his frustrations. However, after deciding they both needed to move on, the two bid their goodbyes before Armothy choked the leader to death.
Although since Armothy wasn't a paramedic in life, he failed to judge whether the leader was dead before 'moving on' and Morty's left arm reverted to normal. Once the target started to come to, Rick held Morty's hand and the two finished the job in choking him. Rick then said, "See Morty? Now we're both accountable."