Did the ghost of Samuel really appear to Saul or was it just a demon or something else?
The Bible is clear about what happens to dead people:
KJV Job 14:10,12 But man dieth, and wasteth away: yea, man giveth up the ghost, and where is he?... So man lieth down, and riseth not: till the heavens be no more, they shall not awake, nor be raised out of their sleep.
KJV Ecclesiastes 9:10 Whatsoever thy hand findeth to do, do it with thy might; for there is no work, nor device, nor knowledge, nor wisdom, in the grave, whither thou goest.
What physically happens to people when they die?
KJV Ecclesiastes 12:7 Then shall the dust return to the earth as it was: and the spirit shall return unto God who gave it.
We see there are 2 components to a living person - the dust and the spirit. Comparing it with how God created man we see that the spirit that God gave man is the breath of life
KJV Genesis 2:7 And the Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul.
The Bible clarifies it further:
KJV Job 27:3 All the while my breath is in me, and the spirit of God is in my nostrils;
Job says that the Spirit of God is in his nose
Now if the breath goes back to God, who can bring a dead person alive again?
KJV John 5:28,29 Marvel not at this: for the hour is coming, in the which all that are in the graves shall hear his voice, And shall come forth; they that have done good, unto the resurrection of life; and they that have done evil, unto the resurrection of damnation.
Only God can bring a dead person alive.
When will this happen? The first quotes verse in Job 14 says "Till the heavens be no more". The Bible clarifies it further:
KJV John 11:24 Martha saith unto him, I know that he shall rise again in the resurrection at the last day.
KJV 1 Thessalonians 4:16 For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:
So the dead people are sleeping in the grave and the spirit of life which was in them when alive returns to God. They will be resurrected by Jesus when He returns, if they have died in Him. (Concerning the wicked see Rev 20 for a clear answer without any jargon).
If God only can raise the dead, the question now becomes - is the witch of Endor a servant of God?
No! In fact God was completely against them! A death sentence was pronounced against them.
KJV Exodus 22:18 Thou shalt not suffer a witch to live.
And again, concerning the very act of Saul:
KJV Isaiah 8:19,20 And when they shall say unto you, Seek unto them that have familiar spirits, and unto wizards that peep, and that mutter: should not a people seek unto their God? for the living to the dead? To the law and to the testimony: if they speak not according to this word, it is because there is no light in them.
If God is against this, then it follows that it is the work of the devil. All witchcraft and sorcery is from the enemy. This is why wherever Gospel spread one of the effect was destruction of witchcraft and sorcery:
KJV Acts 19:19 Many of them also which used curious arts brought their books together, and burned them before all men: and they counted the price of them, and found it fifty thousand pieces of silver.
Now the question comes - is the devil capable of resurrecting dead people? The Bible says that only God and Jesus can resurrect and give life to people:
KJV John 5:21 For as the Father raiseth up the dead, and quickeneth them; even so the Son quickeneth whom he will.
Why? Because God alone has life. He is called "I AM THAT I AM" meaning He exists by Himself. And this life He shares with all created beings.
KJV 1 Timothy 6:16 Who only hath immortality, dwelling in the light which no man can approach unto; whom no man hath seen, nor can see: to whom be honour and power everlasting. Amen.
KJV Acts 17:28 For in him we live, and move, and have our being; as certain also of your own poets have said, For we are also his offspring.
So, if Satan can't resurrect and the witch was a servant/partner of Satan, who/what was the Samuel that came up?
KJV 2 Corinthians 11:14 And no marvel; for Satan himself is transformed into an angel of light.
Satan can impersonate the angels of heaven! It is a much easy work to impersonate the dead people. So it was an evil spirit which impersonated Samuel and not the actual Samuel who came up, for the devil does not have the power to do so