I’m not too sure myself but it sounds like the saintess had a twin sister when they were children, they summoned a demon to end the plague, and the demon in turn possessed the saintess. The twin sister became an apothecary. The ML is the guard who I think was in love with the saintess, and when they flee from the crown prince, he becomes fuel for the demon’s power, allowing the saintess she possesses to use magic. The ML realizing what’s going on after meeting the twin sister makes a contract to be the demon’s host and set the saintess free.
The demon originally came about when the first king used black magic to bring back his deceased wife. The demon said that in return for bringing the woman back, she would be guaranteed a host and power for all eternity. The demon went on to damage every vessel she possessed due to lack of power and had to continue switching vessels, which eventually is what got her to the saintess.
The problem with the contract that the ML made with the demon is that it permanently bound the demon to him. That’s all I could figure out
am i stupid? wth just happened