In Which there exist two Sombras in the Human world because through some magic, he was split in two both physically and in personality. Like a group of both Human and Equestrian mages in the distant past wanted to kill him in an attempt to stop him from completing an evil plan of his and one of the mages, being Human Radiant Hope, refused to allow the death of her childhood friend and instead cast a spell that split him into two halves and then proceeded to imprison his evil half.

The Good Half of Sombra is part of the Faculty of Crystal Prep Academy while his Evil Half exists as an evil spirit haunting an old mansion that was his former residence. literally under house arrest as he was sealed up and imprisoned within it's walls.

This involves two concepts being

• crossovers and interactions between Earth and Equus happening since the dawn of Human History.
• Humans or ponies in the human world becoming ageless and immortal after exposure to magic after a certain amount of time.

The latter would be the reason why Sombra and Radiant Hope would be still alive in the present and haven't aged as the whole incident and the splitting of Sombra occurred almost a century in the past . . . in the 1920's.

I picked that decade because I envision Human Sombra being one of those Humans in Lovecraft's works who worship dark eldritch deities and sometimes wield powers bestowed upon them by said deities at the cost of their sanity and humanity. So he was a power-hungry madman who dabbled in dark magic to gain true power and due to his exposure to dark magic, went insane and developed psychopathy as a result. This "insane" aspect of Sombra would be prominent in his evil half.

Many stories and other fictional works set in the Cthulhu mythos tend to occurred between the 1920's and the 1930's.

As for the mansion, this would explain why the place didn't collapse and completely rot and crumble away. The large amount of wards placed upon the house as well as the pre-existing Dark magic saturating the structure before said wards kept the structure in a preserved state when it should have crumbled and collapsed in the past. However, the Dark Magic from the manor gave birth to an insidious taint that spread across the land the estate stands upon. Withering the surrounding forest into a dead and rotting marsh.

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Sounds interesting