When you drop to 0 health outside your resting place, you transform into a cloud of mist (as in the Shapechanger trait) instead of falling unconscious, provided that you aren’t in sunlight or running water. If you can’t transform, you are destroyed.
While in mist form, you have health equal to half your normal form’s health, you can’t take any actions, speak, or manipulate objects. You are weightless, have a flying speed of 5 meters, can hover, and can enter a hostile creature’s space and stop there. In addition, if air can pass through a space, you can do so without squeezing, and you can’t pass through water. You have advantage on Reflex and Fortitude saving throws, and you are immune to all nonmagical damage, except the damage you take from sunlight.
While in mist form, you can’t revert to your true form, and you must reach your resting place within 2 hours or be destroyed. Once in your resting place, you revert to your true form. You are then paralyzed until you regain at least 1 health. After spending 1 hour in your resting place with 0 health, you regains 1 health.