It's actually depends on how your perceived it, whether it's a happy ending or not.
It can be a happy ending cause the ml is still alive 500 yrs later (although he's zombie)
It can also be not cause, firsts things first she killed him after all that shit claiming she loves/likes him and reluctant to go back to her og world, and secondly he became a zombie (although idk if he had his consciousness still intact though¯_(ツ)_/¯ )
Well zombie sounds ugly undead is better , he looks the same whole brain functioning better
They live together cus they're both immortal in a way (and the timeline is like 1800s-1900s but with magic)
I love the novel and the ending though finally a Novel where ML is punished and they still end up together.
Don't forget ML is an actual Villain from the start , except he doesn't SA or R**ps FL
Spoiler:Ending
it's not happy ending or sad one in the end she kill her husband with arrow in the neck 500 years later he woke up with body of zombie and she woke up like living immortal.
(Ctto)