Cosmic Void

All the air is pulled out of a 20 foot radius sphere. Each breathing creature that enters or starts its turn in the area must succeed on a Constitution saving throw or have the air sucked out of its lungs and begin suffocating. Otherwise it can hold its breath as normal. 

A suffocating creature can survive for a number of rounds equal to its Constitution modifier (minimum of 1 round). At the start of its next turn, it drops to 0 hit points and is dying, and it can't regain hit points or be stabilized until it can breathe again.

The vacuum has the following additional effects:

• No sound can be created within or pass through the sphere. Creatures are deafened while entirely inside it and can't cast spells with verbal components.
• Flight that relies on wings or otherwise depends on the presence of air is impossible; any such flying creatures fall to the ground within the affected area.
• Any creatures or objects entirely inside the sphere are immune to thunder damage and resistant to fire damage. Nonmagical fires are extinguished.