Death involves the separation of the soul from the body (for the sake of clarity let’s call this the “first” death). The “second death” involves the separation of people from God’s favour (see also Revelation 2:11; Revelation 20:6; Revelation 21:8) and so is descriptive of being cast into the “lake of fire”—where the “fire” is the expression of the eternal wrath of God.
14 Then Death and Hades were thrown into the lake of fire. This is the second death, the lake of fire.