some deep stuff here with differences of opinion among Christians.
It is my understanding that the creature we call satan/devil/adversary today was a powerful archangel who rebelled against God some time in the pre-history of humankind. Scriptures used for this are
Isaiah 14,
Ezekiel 28, and
Revelation 12. There are lots of things I don't understand about these passages. It is commonly taught that 1/3 of the angels joined him in his rebellion and that these creatures are what we know of today as demons or dark angels or evil spirits. It seems according to
Jude 6, 2 Peter 2:4 and
Matthew 25:41 that satan and his hordes cannot (or will not) repent and be reconciled to God but will wind up in the lake of fire for eternity.