10/4/2004: This is not an additional play cost, it is a triggered ability.
10/1/2008: This has the supertype world. This supertype is similar to legendary. The difference is that when a world permanent comes into play, any world permanents that were already in play are put into their owners' graveyards. This is a state-based effect called the "world rule." The new world permanent stays in play. If two world permanents come into play at the same time, they're both put into their owners' graveyards.