Impending Disaster

Oracle Text

At the beginning of your upkeep, if there are seven or more lands on the battlefield, sacrifice Impending Disaster and destroy all lands.

Card Rulings

10/4/2004 This ability does not trigger at all if there are not seven lands on the battlefield at the beginning of upkeep. It checks this number again when it is going to resolve and does not do anything if the land count is below 7.
10/4/2004 The ability is not optional.
6/15/2010 If this card is not on the battlefield at the times its ability resolves, then you can’t sacrifice it. However, you still destroy all lands.