
If you can get past the annoying ass disable ad block popup, this is where I read it: https://arigl.online/manga/mon-qua-trao-tang-nguoi/

!!SPOILER!!
The knight is tricked into the "mission" to go get the Demon King's sword. Surprise! It's of course a ruse to get back at the knight who killed the demon's child. The chapters from 4 to 7 are basically the demon taking on various forms to mind break the knight until it finally succeeds.
And when you you think the knight actually gets away and is saved, SURPRISE!
It's a fever dream and she's stuck in mental purgatory, forever being sexually gratified by the demon who took on the form of the princess because it knows that that's what the knight desires. It basically ends on that note. So as the reader, it's left to assumption that the knight spends the rest of of her life being pleasured by the demon IRL but in her mind, she thinks it's the princess.... Awful awful ending.
The ending of this one is straight awful... ╥﹏╥