Revelation 21:8

But as for the cowardly, the faithless, the detestable, as for murderers, the sexually immoral, sorcerers, idolaters, and all liars, their portion will be in the lake that burns with fire and sulfur, which is the second death.” 

And do not fear those who kill the body but cannot kill the soul. Rather fear him who can destroy both soul and body in hell. 

And these will go away into eternal punishment, but the righteous into eternal life.” 

Then Death and Hades were thrown into the lake of fire. This is the second death, the lake of fire.