Talk:The Underworld/@comment-24010718-20131030161734/@comment-24013733-20131030165635

Hardmode Underworld enemies spawn after defeating a Mechanical boss. If you've previously summoned and defeated a mech boss in that world -- even on another character -- the hardmode enemies will spawn in the underworld.

If you can survive them, the Voodoo Demons will still spawn, though they'll be a bit harder to find. Only way to reset the underworld back to pre-hardmode is with a map editor.