Miasma

When natural disaster occurs, the Miasma left behind can be more destructive and more feared than the event itself. Sinkholes can exude this noxious, magical gas as easily as openings in the earth torn by earthquakes. Vulcanism can spew this noxious vapor into the air, forming clouds that roll down mountainsides warping or even killing living things in the path. Even some geysers or natural springs fed by reservoirs deep underground may be contaminated by Miasma.
Theories about what Miasma actually is and why it is produced abound, and the stories are usually steeped in superstition and religious belief. But to those that investigate it, it's fairly clear that is pure mystical energy that usually sprays into the air as a mist, it reacts to everything it touches chaotically. In essence, it causes mutation.

Detection

Miasma is most dangerous when concentrated near the source, or where it flows into valleys, dips, and hollows and it's in these places the most severe danger lies. Further away, it can be detected as a scent in the air. A successfull DC 13 Intelligence (Nature) or Wisdom (Survival) check can indicate the presence of Miasma.

Exposure

Unfortunately, holding one's breath provides no protection from the Miasma. Any contact with this raw, magical force can have dramatic effects. In an area of Miasma, where it is dense enough to appear as a fog, a DC 13 Constitution saving throw is needed to avoid taking 4d10 force damage and becoming poisoned. A successful save will reduce the damage by half and avoid the condition.
If no way out of the Miasma is found, then the above exposure recurs every minute.
When the poisoned condition is removed, the Miasma's warping effect is incurred. See the below table for possible results. If the victim of Miasma is a magical creature or has the ability to cast a spell, a second Constitution save must be made or the character drops immediately to 0 Hit Points and begins dying. The bodies of victims killed by the Miasma mostly burn away, leaving only a few small crystals behind. Magical equipment carried is destroyed, converted to Miasma ash, drust, that mostly blows away. Nonmagical, nonliving items are generally unaffected. If the magical creature or spellcaster survives this second Constitution roll, then they can roll on the Miasma Effects table and pick up a new, additional mutation.

Duration

Miasma will eventually condense, given a few hours. Although it can move with the wind, it doesn't behave like normal gases and fogs, stubbornly lingering in some areas, and often refusing to dispurse regardless the strength of the wind. It seems to lose energy over time, congealing into a very fine, very light dust. This is in fact concentrated magic, mana that can be collected and used in lieu of actual spell components. But it is difficult to gather and dangerous to do so, unless one takes precautions to protect the skin and keep from breathing it in. Over exposure to this drust has the same effect as initial Exposure. So, while extremely valuable, it is also very difficult to safely collect.
It's not well known that the crystals called magicite that are sometime found mixed in with the drust left behind by Miasma are the magically transmuted remains of living beings. Magicite can also be used in lieu of material components. Further, certain spells that can restore life to the dead only function when empowered by miasma crystals.

Type

Metaphysical, Supernatural