It evolves at level 22 while holding either a Burn Heal or Ice Heal into Noxial, which then evolves into Fumighast starting at level 36 while holding an Ice Heal or a Burn Heal.
The Phlask evolutionary line originally contained UFOs in their design.
The Diloweed, Phlask and Vectol lines were the last evolutionary lines to be confirmed.
The final design for Noxial was made by a different artist than the designs for Phlask and Fumighast. Both artists also created designs for the other Pokémon in this evolutionary line, which lost to the other artist's design(s).
The rejected Fumighast design (which went with the confirmed Noxial) had two heads, and required evolving both forms of Noxial together in order to evolve.