Planar Collapse

Enchantment, 1W (2)
At the beginning of your upkeep, if there are four or more creatures on the battlefield, sacrifice Planar Collapse. If you do, destroy all creatures. They can't be regenerated.¹
With heavy heart, Urza doused one world's light to rekindle another's.
Illus. Mark Zug

Gatherer Card Rulings [Nov 2009]

• 10/4/2004: If this card is not on the battlefield when the ability resolves, then you can't sacrifice it and it will not destroy any creatures. The sacrifice is a cost paid during resolution.
• 10/4/2004: This ability does not trigger at all if there are not 4 or more creatures on the battlefield. It also checks this at the start of resolution and does nothing if this is not still true.
• 10/4/2004: The ability is not optional.

Format Legality	Legal in Vintage, Legacy Banned in Extended, Standard, MTGO Classic
Printings (1/6)	Urza's Legacy (Rare)