Source: Plane Shift - Ixalan

Prerequisite: Vampire (Ixalan)

As an action, you can transform the lower half of your body into an inky black vapor, allowing you to float through the air. While transformed, you have a flying speed of 30 feet. You can maintain this form for up to 10 minutes. Once you use this ability, you can't use it again until you finish a short or long rest.