Have you ever seen a person sleep soundly?

It is somewhat like they are in a state of death, particularly when their eyes are closed and their chest so subtly rises up and down almost imperceptibly as they otherwise lay motionless.

If you were to gently open the eyes of a person sleeping very soundly and position yourself so that the sleeper appears to be staring directly at you, it would be obvious to you that the sleeper's eyes were not seeing nor noticing you; it would be as if they had had a brain lobotomy: nothing would register in their brains as to having any response to what otherwise would be quite visible to their conscious mind and normally responded to. So the sleeper sees, but yet does not see. Perhaps also even though the ears of a sound sleeper work, soft sounds do not register in the brain of the deep sleeper.

At least looking directly into the eyes of a gorilla point-blank behind protective glass cage, the ape notices motion and movement it might or might not respond to, depending upon the momentarily whims of its instincts. But there is no love nor hate nor affection nor companionship in those animal eyes made not in the morally-perceptive and body/spirit/soul image of God but rather mere body and spirit. Again, it is like the animal's brain has had a lobotomy - disabling it from exhibiting human understanding.

With that in mind, it is somewhat understandable that the Old-Testament authors recorded the dead as "sleeping with their fathers" plus Jesus and other New-Testament people sometimes and/or temporarily referred to certain dead ones as "sleeping" who needed to be and/or will be "awakened."

It has been Scripturally recorded that Moses died, though his mysterious burial place was strangely never found [Deuteronomy 34:6].

There are some individuals mentioned particularly in the Old Testament which have VERY little info given about them.

Two in particular are found, the first of which is recorded in Joshua 5:13-15 as follows.......

13 When Joshua was by Jericho, he lifted up his eyes and looked, and behold, a man stood before him with his drawn sword in his hand; and Joshua went to him and said to him, "Are you for us, or for our adversaries?"
14 And he said, "No; but as commander of the army of the LORD I have now come." And Joshua fell on his face to the earth, and worshiped, and said to him, "What does my lord bid his servant?"
15 And the commander of the LORD's army said to Joshua, "Put off your shoes from your feet; for the place where you stand is holy." And Joshua did so.

........and the second of which is recorded in Daniel 3:24-27 as follows:

24 Then King Nebuchadnezzar was astonished and rose up in haste. He said to his counselors, "Did we not cast three men bound into the fire?" They answered the king, "True, O king."
25 He answered, "But I see four men loose, walking in the midst of the fire, and they are not hurt; and the appearance of the fourth is like a son of the gods."
26 Then Nebuchadnezzar came near to the door of the burning fiery furnace and said, "Shadrach, Meshach, and Abednego, servants of the Most High God, come forth, and come here!" Then Shadrach, Meshach, and Abednego came out from the fire.
27 And the satraps, the prefects, the governors, and the king's counselors gathered together and saw that the fire had not had any power over the bodies of those men; the hair of their heads was not singed, their mantles were not harmed, and no smell of fire had come upon them.

The one to whom Joshua bowed down to, as the fourth one who Nebuchadnezzar referred to as the son of God, perhaps were theophanes (i.e. Old-Covenant appearances of Christ the Son of God before He became incarnate as a babe in Mary's womb). Be that as it might be, not much more identity is given either individual.

Genesis 5:22 Enoch walked with God after the birth of Methuselah three hundred years, and had other sons and daughters.
Genesis 5:23 Thus all the days of Enoch were three hundred and sixty-five years.
Genesis 5:24 Enoch walked with God; and he was not, for God took him.
Hebrews 11:5 By faith Enoch was taken up so that he should not see death; and he was not found, because God had taken him. Now before he was taken he was attested as having pleased God.
Jude 1:14 It was of these also that Enoch in the seventh generation from Adam prophesied, saying, "Behold, the Lord came with his holy myriads....

2 Kings 2:1 Now when the LORD was about to take Elijah up to heaven by a whirlwind, Elijah and Elisha were on their way from Gilgal.
2 Kings 2:11 And as they still went on and talked, behold, a chariot of fire and horses of fire separated the two of them. And Elijah went up by a whirlwind into heaven.

So it seems that neither Enoch nor Elijah, who were non-perfect mortal men, never died. Yet? Revelation 11:3-12 states this:

3 And I will give power unto my two witnesses, and they shall prophesy a thousand two hundred and threescore days, clothed in sackcloth."
4 These are the two olive trees and the two candlesticks, standing before the God of the earth.
5 And if any man will hurt them, fire proceedeth out of their mouth and devoureth their enemies; and if any man will hurt them, he must in this manner be killed.
6 These have the power to shut heaven, that it rain not in the days of their prophecy, and have power over waters to turn them to blood, and to smite the earth with all plagues, as often as they will.
7 And when they shall have finished their testimony, the beast that ascendeth out of the bottomless pit shall make war against them, and shall overcome them and kill them.
8 And their dead bodies shall lie in the street of the great city, which spiritually is called Sodom and Egypt, where also our Lord was crucified.
9 And they of the people and kindreds, and tongues and nations shall see their dead bodies three days and a half, and shall not suffer their dead bodies to be put in graves.
10 And they that dwell upon the earth shall rejoice over them and make merry, and shall send gifts one to another, because these two prophets tormented those who dwelt on the earth.
11 But after three days and a half, the Spirit of life from God entered into them, and they stood upon their feet; and great fear fell upon those who saw them.
12 And they heard a great voice from Heaven, saying unto them, "Come up hither!" And they ascended up to Heaven in a cloud, and their enemies beheld them.

So if the two witnesses turn out to be Enoch and Elijah - who might reappear as quickly and/or as uniquely as they disappeared - they too will experience the lethal fate of all mortal humans.

As you might notice from a careful reading of particularly the Old-Testament reference of Genesis 14:18-20 and apparent time-frame context after that reference, Melchizedek met Abram while ABRAM was yet called Abram and not yet renamed Abraham by God later on [Genesis 17:5]. Yet, the New-Testament book of Hebrews never refers to "Abram" - but always merely: "Abraham" - when referring to that same person in the New-Testament book of Hebrews.

That seems to be a perplexing discrepancy or even apparent error on the part of the writer of the book of Hebrews.

However, the "Abram" who met Melchizedek is the same person as the one who was renamed by God "Abraham" [later on, according to contextual timeframe?].

It says that tithes were received by mortal men, but it is testified that Christ [Jesus] became the High Priest of the confession of our faith not because of a legal requirement pertaining to bodily descent, but who lives eternally because of His power of an indestructible life" [Hebrews 7:16], which Divine High Priest (Who lives continually even up to the present) obtained tithes from Melchizedek himself.

Christ Jesus - not a Levitical Aaronic priest but rather descended from the humanly-priestless tribe of Judah [Hebrews 7:14] - has no need to engage in mutual priestly tithing (that is, one priest tithing to another priest) [Hebrews 7:9].

According to Hebrews 7:23-28, former priests (both Levitical/Aaronic and Melchizedek himself) "were prevented by death from continuing in office", but Christ Jesus continues holy, blameless, non-stained, separated-away-from-sinners, exalted-above-the-heavens (thus very dissimilar to Melchizedek): HIGH Priest permanently because "He continues forever...and always lives" - NOT like imperfect, weak, needing-daily-to-sacrifice-for-his-own-sins Melchizedek.

Genesis 14:18-20 neither states nor implies that Melchizedek was or is immortal (remaining alive to the contemporary time of the human author of the book of Hebrews, nor up to the present time).