Lethal Vapors, and Pithing Needle

Prerequisites

• Lethal Vapors on the battlefield
• Pithing Needle on the battlefield
• "Lethal Vapors" chosen with Pithing Needle

Steps

1. Whenever a creature enters the battlefield, Lethal Vapors triggers, destroying that creature
2. Because Pithing Needle prevents Lethal Vapors's activated ability from being activated, it cannot be destroyed due to its ability

Results

• Destroy all creatures that enter the battlefield