Something's Rotten in Faerun
Potion of Gaseous Form
By imbibing this magical liquid, the individual causes his body, as well as anything he’s carrying or wearing, to become gaseous. The gaseous form is able to flow at a base speed of 3/round. (A gust of wind spell, or even normal strong air currents, will blow the gaseous form at air speed.)
The gaseous form is transparent and insubstantial. It wavers and shifts, and can’t be harmed except by magical fire or lightning, which do normal damage. A whirlwind inflicts double damage upon a creature in gaseous form. When in such condition the individual is able to enter any space that is not airtight—even a small crack or hole that allows air to penetrate also allows entry by a creature in gaseous form. The entire potion must be consumed to achieve this result, and the effects last the entire duration (4+1d4 turns).