Unbreathing creatures do not require air to survive. These include undead, nonliving, and plant-type monsters. Player characters can achieve this by playing a ghoul, vampire (regardless of satiation, they do not have to breathe), mummy, gargoyle, grey draconian, or by being in Ice Form, Statue Form, Lich Form, Tree Form, Fungus Form, or Wisp Form.
Because these must be inhaled to affect their victim, unbreathing creatures are unaffected by mephitic or poisonous clouds, but are susceptible to other harmful clouds (such as flaming, freezing, mutagenic, or miasma). Unbreathing creatures are also immune to asphyxiation damage (such as through curare needles or water elementals), and take reduced damage from Primal Wave. Note, however, that unbreathing creatures can still drown in deep water (with the exception of grey Draconians, who specifically have the ability to move through deep water, albeit slowly and awkwardly).