Atmospheric pressure

The air pressure in a 16ft sphere gradually increases, over the course of three turns. Once these turns are over, any creatures that begin or end their turn within the area take 2d4 damage on a failed CON saving throw, or 1d6 on success. They are not informed of the source of this damage, or how it is inflicted. In order to detect the sphere, creatures must make a DC 10 perception check after taking damage from it. The sphere cannot be physically destroyed, but fades away after three rounds. This spell is only effective in locations that have a gaseous atmosphere.