Myson

Myson makes an appearance in Yuri and Constance's shared Paralogue, A Cursed Relic, where he performed several experiments in secret. Learning that a Relic had been acquired by Duke Gerth, he led a group of Agarthans to steal the Relic, and used the results of his experiment to forcefully control a group of thieves. One of these thieves attempted to attack Gerth, only to be killed and revived as a Demonic Beast. He was surprised by the appearance of Byleth and their students, considering them an "unexpected obstacle", but was greatly pleased by the results of his experiments.

Despite the assistance of the Demonic Beasts, Myson was defeated by Byleth and their students. He expressed disappointment in not being able to obtain the Hero's Relic, but was content enough with the results of his experiments and withdrew. The Relic that he had intended to steal were the Fetters of Dromi.

Myson appears in the Azure Moon route. Despite their leader's death at the hands of Kingdom forces, Myson and several agents of those who slither in the dark remained under Edelgard's service, although due to their conflicting beliefs they were only there to assist her in taking out their common enemy.

Prior to the battle, Ingrid (or Gilbert if Ingrid was not deployed) notices that several of the Imperial mages weren't wearing their typical uniforms. If Myson is killed, the remaining agents will retreat from the palace, unwilling to sacrifice their goal attempting to defend Edelgard.

He also appears in Chapter 21 of the Verdant Wind route, along with others of Those Who Slither In The Dark.

In the Silver Snow route, he appears in Chapter 20 alongside others of Those Who Slither In The Dark.