Could you perhaps spoil some for me?
The impression I've always gotten, from the very start, is that her soul is forcefully resisting and rejecting the necromancy. Either because she simply wants to stay dead and enjoy that "peace" or what I feel is more likely that she's rejecting his current path and doesn't want to do it, letting the world move on from both of them or something like that.If he cannot revive her, there are a few things I suspect.
One is because of the protection of whatever Gods they worship, her soul, apart from her remains, cannot be affected by Necromancy because of their protection or blessing.
The second which I think is more likely is, that her soul has already moved on. Dwight can no longer use Necromancy to call for her soul, because she's already back as someone else.
A lot can happen in the time it took for Dwight to become a Lich.
Ngl but I kinda expected that to happen. But the second half was a surprise.At the ending of the web novel he manages to bring the hero back. She didn't want to be revived since she did what she had to do during her life. She's happy he found something to work towards, but disagrees with his methods. She challenges him for a duel to death because she's the hero and he's the demon lord. He kills her, she tells him to not resurrect her again and just keep living and doing what he's doing.