Death and hell came into the world through the fall of Adam. “Death” refers to physical death, or the separation of the body from the spirit; “hell” refers to spiritual death, or being cast out of the presence of God. Both death and hell would have force upon our souls for eternity if it were not for the atonement of Jesus Christ. That is why the Book of Revelation explains that the Savior held the keys of death and hell, I am he that liveth, and was dead; and behold, I am alive for evermore, Amen; and have the keys of hell and of death (Rev 1:18).
Through the atonement death and hell are overcome. Death is overcome for all who live on the earth. Hell is overcome for all but the sons of perdition (DC 76:43-44). Even those of the telestial kingdom will be redeemed in the due time of the Lord (DC 76:31-44). They also are blessed with the ministration of the Holy Ghost and are therefore not cast out of God’s presence forever (DC 76:86). For the sons of perdition, it is a different story. They are cast out of the presence of God to rule with the devil and his angels in eternity (DC 76:44).
So, for all but the most wicked—because they were not willing to enjoy that which they might have received (DC 88:32)—mankind is saved from the grips of that awful monster. Paul taught that the Lord would reign, till he hath put all enemies under his feet. The last enemy that shall be destroyed is death (1 Cor 15 26). John makes it clear that Death and Hell are overcome at the same time when the Savior casts them into the lake of fire:
And I saw the dead, small and great, stand before God…and the dead were judged…And the sea gave up the dead which were in it; and death and hell delivered up the dead which were in them: and they were judged every man according to their works. And death and hell were cast into the lake of fire. This is the second death (Rev 20:12-14).