Psalm 68:18 Thou hast ascended on high, thou hast led captivity captive: thou hast received gifts for men; yea, for the rebellious also, that the LORD God might dwell among them.
Ephesians 4:8 Wherefore he saith, When he ascended up on high, he led captivity captive, and gave gifts unto men.
'Leading captivity captive’ has to be understood in the light of Isaiah 61:1 and Luke 4:18.
Isaiah 61:1 The Spirit of the Lord GOD is upon me; because the LORD hath anointed me to preach good tidings unto the meek; he hath sent me to bind up the brokenhearted, to proclaim liberty to the captives, and the opening of the prison to them that are bound;
Luke 4:16 And he came to Nazareth, where he had been brought up: and, as his custom was, he went into the synagogue on the sabbath day, and stood up for to read. 17 And there was delivered unto him the book of the prophet Esaias. And when he had opened the book, he found the place where it was written, 18 The Spirit of the Lord is upon me, because he hath anointed me to preach the gospel to the poor; he hath sent me to heal the brokenhearted, to preach deliverance to the captives, and recovering of sight to the blind, to set at liberty them that are bruised,
All men (and women) are held captive in that they are under the sentence of death for sin. Christ has provided a way of escape from sin and death and has effectively opened the prison of those who were bound (i.e. to those who believe and obey his gospel).
Jesus now has the keys of the grave and of death.
Revelation 1:18 I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell (hades – the grave) and of death.
At the appointed time he will return to open the graves and restore life to the dead.
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: 17 Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
In a figure Christ has taken captive the captivity. He now has power over the prison of sin and death to release the captives. In confirmation of this, the Holy Spirit gifts were given after his ascension to God’s right hand.
The Old Testament saints are still in their graves; they will not receive the reward of eternal life without us – that is: until Christ returns.
Hebrews 11:13 These all died in faith, not having received the promises, but having seen them afar off, and were persuaded of them, and embraced them, and confessed that they were strangers and pilgrims on the earth... 39 And these all, having obtained a good report through faith, received not the promise: 40 God having provided some better thing for us, that they without us should not be made perfect.
Those who died in faith before Christ came and those who die in faith after are all to be rewarded at the same time.
I hope you have found this helpful.