World Enchantment
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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.
If a permanent enters as a copy of another permanent, the permanent it is copying is destroyed and the copy stays on the battlefield.
Since phasing in does not cause enters abilities to trigger, it is possible to phase in a card which is already on the battlefield and not trigger this effect.