Plot booster

A plot booster is a concept used in Legacy-style board games, being an unknown element added to a certain game and thereby changing certain rules that players are used to.

In Magic
In connection to Magic: The Gathering, a plot booster named "Uncovered Cavern" was first mentioned on the test card Planequake in the Mystery Booster set. No such booster pack is part of that set in reality, much like Contraptions weren't part of Future Sight though they were mentioned on.

it may be assumed that if the plot booster would be opened, its mysterious contents could be added to a game of Magic while it is in progress, changing the playing environment drastically. A plot booster can only be opened if certain circumstances are met.

Rulings

 * If you don't have the appropriate unopened plot booster when instructed to open one, that part of the spell or ability's effect is ignored.
 * Creatures and planeswalkers dealt lethal damage by Planequake won't be destroyed until state-based actions are performed, after you've opened the "Uncovered Cavern" plot booster if applicable.