Solid fog

When solid fog is cast, the wizard creates a billowing mass of misty vapors. The caster can create a rectangular or cubic mass at least ten feet on a side. The fog obscures all sight, normal and infravision, beyond two 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 very slowly.

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.