Some creatures have the supernatural or spell-like ability to take the form of a cloud of vapor or gas.
Creatures in gaseous form can't run but can fly. A gaseous creature can move about and do the things that a cloud of gas can conceivably do, such as flow through the crack under a door. It can't, however, pass through solid matter.
Gaseous creatures can't attack physically or cast spells with verbal, somatic, material, or focus components. They lose their supernatural abilities (except for the supernatural ability to assume gaseous form, of course).
Creatures in gaseous form have damage reduction 10/magic. Spells, spell-like abilities, and supernatural abilities affect them normally. Creatures in gaseous form lose all benefit of material armor (including natural armor), though size, Dexterity, deflection bonuses, and armor bonuses from force armor (for example, from the mage armor spell) still apply.
Gaseous creatures do not need to breathe and are immune to attacks involving breathing (troglodyte stench, poison gas, and the like).
Gaseous creatures can't enter water or other liquid. They are not ethereal or incorporeal. They are affected by winds or other forms of moving air to the extent that the wind pushes them in the direction the wind is moving. However, even the strongest wind can't disperse or damage a creature in gaseous form.
Discerning a creature in gaseous form from natural mist requires a DC 15 Spot check. Creatures in gaseous form attempting to hide in an area with mist, smoke, or other gas gain a +20 bonus.