Solid fog

When solid fog is cast, the wizard creates a billowing mass of misty vapors similar to a wall of fog spell. The caster can create less vapor if desired, as long as a rectangular or cubic mass at least 10 feet on a side is formed. The fog obscures all sight, normal and infravision, beyond 2 feet. However, unlike normal fog, only a very strong wind can move these vapors, and any creature attempting to move through the solid fog progresses at a movement rate of 1 foot per round. A gust of wind spell cannot affect it. A fireball, flame strike, or wall of fire can burn it away in about a minute.

Note: This version of solid fog originally appeared in the 2nd Edition Player's Handbook for Advanced Dungeons & Dragons from TSR. Its use here is for the purposes of providing context for the campaign only.