Planar Collapse

Enchantment, 1W (2)
At the beginning of your upkeep, if there are four or more creatures in play, 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

Crystal Keep Rulings Summaries

• This ability does not trigger at all if there are not 4 or more creatures in play. It also checks this at the start of resolution and does nothing if this is not still true. [D'Angelo 1999/05/01]
• The ability is not optional. [Barclay 1999/02/09]
• If this card is not in play when the ability resolves, then you cannot sacrifice it and it will not destroy any creatures. The sacrifice is a cost paid during resolution. [WotC Rules Team 1999/03/18]

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