Spoiler Alert
The cousin decides to do the experiment to gain power and upstage Ivona during the last part of the fall expedition. Instead, Ivona summons the king of divine beasts and Tristan's summons are completely forgotten about. After the expedition, he talks with the head of the church (ultimate villian besides the demon king, of course). Head priest offers to heal him so he isn't in pain, which Tristan accepts but the priest heals him in a way where Tristan can no longer speak. This way Tristan can't talk about the priest's misdeeds. Tristan is also still scarred from the experiment since those wounds cannot be healed, only the extra power/pain from the extra power can be taken away or healed from the experiment user. Tristan is in dismay because he says he was only confident about his looks and now he has lost those and his voice. The priests henchmen through him out on the streets where he presumably lives out his remaining days as a beggar. Asshole does a lot of stuff that gets him in trouble, promising to summon the divine beast and failing then setting fire to the military supplies to make an escape, teaming up with the head priest to try to kill Ivona, ect. Ultimately, he is disowned from his family, caught and charged for his crimes and executed by guillotine. We see Tristan in the crowd watch his execution and that's the last either of them are mentioned. If you're curious, head priest is ultimately killed by the demon king by trying to make a deal with him. Like dude, demon king ain't gonna make a deal with a human, he doesn't give a shit. Anyways, everything wraps up into a happy ending, they have a daughter together and the princess becomes the first empress with red haired lady as her ambassador.
Someone give me the spoils on the cousin and the asshole pls I CANNOT reread this again (๑•ㅂ•)و✧