Closing Walls

When Closing Walls enters the battlefield, put X shift counters on Closing Walls, where X is how many creatures are on the battlefield. Remove 1 shift counter on your upkeep. When there are no more shift counters on Closing Walls, sacrifice it
and destroy all creatures on the battlefield.
"Don't panic! If we stay calm, we can find a way out!"
— Renald the Adventurer, last words