Planar Collapse
Enchantment
At the beginning of your upkeep, if there are four or more creatures on the battlefield, sacrifice this enchantment and destroy all creatures. They can't be regenerated.
With heavy heart, Urza doused one world's light to rekindle another's.
Mark Zug
Urza's Legacy (ULG)
• #18Rulings
If this card is not on the battlefield when the ability resolves, then you can’t sacrifice it. However, you still destroy all creatures.
(wotc • 2010-06-15)
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.
(wotc • 2004-10-04)
The ability is not optional.
(wotc • 2004-10-04)
Rulings
If this card is not on the battlefield when the ability resolves, then you can’t sacrifice it. However, you still destroy all creatures.
(wotc • 2010-06-15)
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.
(wotc • 2004-10-04)
The ability is not optional.
(wotc • 2004-10-04)