Nether Void

• WWizards of the Coast2004-10-04
  Official Ruling
  This is not an additional cost, it is a triggered ability.
• WWizards of the Coast2008-10-01
  Official Ruling
  This has the supertype world. When a world permanent enters, any world permanents that were already on the battlefield are put into their owners' graveyards. This is a state-based action called the "world rule." The new world permanent stays on the battlefield. If two world permanents enter at the same time, they're both put into their owners' graveyards.
• WWizards of the Coast2009-10-01
  Official Ruling
  The ability triggers whenever any player (including you) casts a spell of any type.

Rulings via Scryfall