The messiah first appears during the summer festival, during an attack on humanity by the monster rats. Being the ultimate weapon for the monster rats, since humans cannot kill another human, as it would result in their own deaths thanks to their death feedback, the messiah is used to wipe out every human in the area of the festival.
Ultimately, he is killed by his death feedback, triggered by a trap Saki, Satoru and Kiroumaru provide - Since the messiah believes he is a monster rat, the three of them disguise Kiroumaru as human and send him out for the messiah to kill. He quickly realizes the mistake he made, as soon as the disguise of Kiroumaru falls off and reveals that he is actually a monster rat. The death feedback kills the messiah, erasing Squealer's ultimate weapon to overthrow humanity