At the beginning of each player's upkeep, that player may sacrifice a creature of their choice. If that player doesn't, they lose 5 life and you sacrifice Pillar Tombs of Aku.
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.