I'll try but English is not my first language so while I was reading the novel ( btw it's verrry long ) I was concentrating at the main events but anyway ...
Spoilers for death is the only way for villainess:
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It was said in the novel that Eclise is the prince of a ruined country or something like that be he was caught and brought as a slave so when the FL bought him he knew that she has something on her mind not pity or something and as the time goes on he was sure that she needs him in whatever she wanna do but then he brought the real Yvonne ( the heroine ) to the duke's mansion. Our MC feels so betrayed by his actions so she decides to act like he doesn't exist aka she hated him so much and he was sent to prison by the duke because he was suspicious. Anyway Penelope visits him and he starts acting all yandere like I can't live without you and please don't take of the ring (the ring she was wearing is a thing used to discipline the slave by giving the him a electricity shocking remeber the slave auction in thr manga they give it to her then)
Sorry I pressed post by accident ( ̄∇ ̄")
Anyway he begged her not to take the ring and he doesn't mind whatever she does to her and that he wants her like he went insane and penelope was so pissed that she finally understood that eclise is a yandere psychopath so she took of the ring and smashed it infront of him and told him she will never see him again and he just continued to look at the way she left for hours and stretching his hands untill it bled
Anyway in the final battle in the manga he came as a villain with yvonne and tried to do the same thing again he did in the prison he abducted her and things happined anyway she was getting attacked by a dragon and he saved her with his life so he dies while telling her that he loves her (〜 ̄△ ̄)〜
Well it may sounds romantic I mean his ending but to me it was seriously creepy (⊙…⊙ )
letio is kinda like eclise o_o