Spoilers ahead! Warning tho, it's a lot. =w='
Basically; long time ago, there was the original head of the Lapireon family, Einhard lapileon. That's the picture of him in the locket. He was in love with a girl, Lira, who was either Pereshati ancestor or similar to her. All you really need to know is that Einhard loved her a lot, like how currently Theo loves Pereshati.
Someone didn't like that. That someone was a witch who was obsessively in love with Einhard; which was unrequited. He had his own love and she was INSANE. So, crazy witch then cursed the family with the now famous and deadly blood curse; the duke's lover confronted the witch, saying she'll die if it means lifting the curse. It was a trick. The duke's love died, the curse continued, and the witch now stupidly thought she had the duke's love finally... But he committed suicide, saying he would kill the witch in his next life.
Because the curse came with a witch vow, she was then cursed with being immortal; that now crazy witch is in the princess body. It's not her living forever in her same body, but cursed to continue living and dying on different bodies since then. Hence why she's in the princess body and not her original. Also, she doesn't even really see Theo, as Theo; just her 'beloved' Einhard. Crazy witch is just treating him as a replacement too. (╬￣皿￣)
The original princess wasn't this loved-obsessed psycho either; she was a normal girl, who was very sick. It was just that she had died due to said illness (I think, it's a little fuzzy) and now the crazy witch inhabits her body thanks to her own curse. Hence why there was talk of the princess at the beginning, never leaving her room and having health problems in the previous life.
Now, the curse it's self is easy to stop and lift. You just have to kill the witch.
Four of the extended family of the Lapireon family, then stupidly makes a deal with the witch, a witch vow. Whoever breaks it, they die a most horrible and painful death. It just triggers suddenly, no warning either. The witch/princess tells them to bring her Pereshati as a sacrifice, then the curse will be lifted. (The same trick she used before with Lina, which sadly worked then.) However, Pereshati herself comes and makes a deal/vow with the witch; she'll die for the family, so long as not one of them is harmed or hurt and the curse IS lifted. (the close family btw, not the traitor extended family.)
The witch then starts murdering people within the palace, and I mean A LOT; becoming a bloody and horrifying massacre. The witch is taking their energy by killing people (I believe, because her now princess body is sickly, she kills/sacrifices people to keep her healthy and to gain back her witch powers. I don’t think her normal non-witch princess body could muster strength to kill Pereshati. This is my guess however.) and right as she's about to kill Pereshati, Theo comes rushing to the rescue, before collapsing himself. (the curse at this point is killing him, hence why Pereshati confronted the witch to stop and lift the curse, before it kills him)
Expect all works out and we get happy ending, because the witch had broken the vow. The witch promised to not harm the Lapireon family, she broke it already without knowing, by trying to hurt Pereshati. Who IS apart of the Lapireon family. (In her own crazy delusion, she never considered Pereshati a member of the family; her own grave mistake) She ALSO then realizes the other vow; the one made with the four extended family members? That's also broken. Because they didn't bring Pereshati themselves, she came herself willingly.
And because they broke the vow, both the witch and traitorous Lapireon family members die a very, VERY painful death.
Btw! Pereshati knew about the witch’s trick/lie, that if ‘she only dies the curse will be lifted.’ Pereshati just played along in a way, playing her own trick with the vow to not harm the family. To trick the witch to essentially kill herself and truly end the curse.
Side note: Theo also becomes emperor! The whole royal family weren't painted in a good light thanks to the witch/princess. With her cult and sacrificing people to restore her power and the emperor being somewhat aware of it all and still covered it up. And when Pereshati shows up to be a 'sacrifice,' because the blood curse suddenly takes a turn for the worst, the witch/princess goes on a rampage, killing all the servants in her palace and abuses her ‘royal authority’ to keep the royal guards from intervening. All of this corruption is what gave the Lapileons cause to remove the royal family from power.
…..Also when Theo came to rescue Pereshati, the emperor made an entrance during this very heated and intense confrontation. The witch is trying to kill Pereshati after all! But uh... The emperor tries to accused/order Theo to protect his daughter, but he ends up being stabbed by her instead because he tried to interfere…...... Whoops? ( ￣□￣||)
Lemme know if you gots any questions (=・ω・=)