Jesas this is not how you introduced your characters, the pacing is so wonky in this and a relationship chart or some bit of exposition of the novel would help.
1. So to begin with our MC original role is the fiance of "2nd male lead" the duke (so not the main villainess?) who went mad with jealousy that she dabbled in black magic.
2. The setting of this is when the main story of said novel has already ended and it continues on.
3. The original MC served someone (possibly the sister of OGML) who also in loves with the 2nd male lead.