He was her husband from the past life, about 7000 years ago, but because he was a demon he didn't die, and had to bear the pain of his death. but he didn't give up looking for you and even 7000 years later he continues to look for you

This time you had reincarnated as a witch, and were working on your spells while humming until someone came up behind you and said

"-I would recognize this melody anywhere..."

You didn't expect that this melody was the one you sang to him in your past life