MikXL_23 What if when the old man reincarnates into your fantasy world, he becomes younger?
Bear with me here. The idea is forming as I'm writing this.
He becomes younger but his mind is slowly recovering from being an old man. During his past life, he had Alzheimer's Disease, slowly forgetting and losing control of his body. With his new body, his memories are slowly returning but they come in scattered fragments, flashes of another life.
In the new world he meets his wife. In his first life, his wife left him early on in their marriage. Her life taken by an unfortunate accident when a tsunami struck their home. Of course, in her new life his wife has no memories of him nor their past. Yet, he still loves her. For better or worse, she was the only one he ever truly loved.
They spend their days happily falling in love once again. His second shot at living a full, happy life with her... is what he wanted but the will of the gods are not so benevolent. He was called there, summoned through a horrible ritual of tearing a soul from one body and replacing it with another.
A plaything for the gods, so to speak.
Disasters after another struck the small village he resided in. Floods, earthquakes, etc. Packs of wild beasts and monster razed houses, killing dozens. Caught in the crossfire, his wife is swept up in the game of gods.
Left alone once more, his heart shattered, our MC has once again lost his loved one to powers beyond him. He finds her bloodies body among the ruins of their village. Her smile sad and weak, one hand gingerly placed over her stomach. In this time of despair, more fragments of his mind return. A soft lullaby. Sweet, soothing humming filled a quiet room. Sitting there on a familiar chair was his wife, her belly a bulge. His baby. Their baby.
Twice, he's lost his wife. Twice, their unborn child has been killed. Twice, he was powerless to stop it.
Piqued by the circumstances of their new plaything, one god decided to have a little more fun. Mischievous and cunning, the god approaches the MC with a promise of power in exchange for a good show. The god also wishes for the demise of other gods but MC has already vowed to kill every god or goddess which makes the god who approached him more amused.
"Your hunger for revenge, the hatred and animosity welling up within you, I shall turn it into your strength. Now quickly, grow and kill us all. I'll be waiting."
To call it a blessing or a curse, the power bestowed onto him was something that could not be contained within a human vessel, morphing him into a powerful demon. The raw power inside threatened to burst free at any moment, twisting his mind and losing himself to insanity.
In that state of self torment, pain and anger, hundreds of year passed by. His demonic body unable to age. His mind no longer sane.
.
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Darn it. I wrote too much. Originally it was just going to be a general idea but then it kept growing and... yeah. In my mind I already have a couple of ideas of how it could end. Take my brain barf. Even now I'm stopping myself from continuing since I'm just supposed to give an idea, not a whole story in itself. I don't care if you do/don't use any of ideas I just wrote down.
Your post piqued me, much like the god in my idea, that is all.
Hopefully this helped?