Reverend Henry Kane

Reverend Henry Kane is the principle villain of the Poltergeist series (although in the original movie he was simply named "The Beast") - in life he was a cult leader who held an apolcayptic view on the world, sealing himself and his followers inside a cave system to die: this location would eventually be built over to make the home the Freeling family would reside in. This origin story is in contradiction with the original movie's plot but seems to be the official story of the sequels.

Due to Kane's evil deeds he became the "Beast" and held the souls of his followers prisoners in the afterlife and feed off their power to make himself a demonic force: Kane targetted the young Freeling daughter (Carol Anne) due to her innocence and famously kidnapped her, leading to the confrontation that made up the events of the original Poltergeist movie - eventually Carol Anne was rescued from Kane's clutches by her determined mother and father though in the end Kane and the other spirits imploded the entire house as a final show of power

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(note: as I have not watched the sequels enough to be confident on the plots I shall end this here but please add to this if you know more, thank you)