Rephaim - Summarised in 30sec!

“The Bible affirms that the Rephaim tribes were physical giants, who originated from within groups of Amorite humans, their demonic spirits having “come out of the (dead) Nephilim,” and that as such there were people groups which featured “Rephaim” as their tribal designation. There were also other tribes of Rephaim giants under different names.
There are places named after the Rephaim, particularly the region of Bashan (“the land of the Rephaim” which can also be legitimately translated as “the hell of the dead”!) and a valley in Israel (“the Valley of Rephaim” or “Pit of Death”). The term is usually translated as “giants.”
The Rephaim giants were named after the spirits within them, the very same spirits that had previously walked the earth in the Nephilim of the pre-Flood world. The physical size of the Rephaim was due to the change brought about by the corruption and defilement of the human host, a leftover trait of the Nephilim.
The Rephaim spirit within a person served as an influencer, additional to that person’s own human spirit, thus creating a kind of duality. When the spirits separated at death, the human spirit was believed to reside with the body, while the Rephaim spirit departed and was released upon the earth. These were subsequently referred to as “unclean spirits,” a term sometimes translated in the NT as “demons.”
The Rephaim gods or Biblical shêdîm (devils) being by nature territorial, often controlled earthly kings in order to take advantage of their sphere of influence. Thus, they are often associated with dead kings, and the living kings claimed connection to them through ancestral or royal heritage and occultic spiritual influence. These Rephaim gods ranged in rank from minor to supreme deities in Canaanite religion.
Because some Rephaim spirits are spirits of the dead (in the case of deified kings), they are frequently connected with the underworld, despite the fact that they are not confined to that world but are free to trespass into ours.
The Rephaim were always regarded as evil, unholy and unclean to Israelites, despite the belief of apostates that the Rephaim had healing powers. The Scriptures tell us that they have a tendency to congregate together.”
Proverbs 21:16 “The man that wandereth out of the way of understanding shall remain in the congregation of the Rephaim.”

Excerpt from T.J. Steadman, “Answers to Giant Questions.” For a much more detailed breakdown of everything discussed here, please consult the book!