It evolves from Phlask starting at level 22 while holding a Burn Heal or an Ice Heal, and evolves into Fumighast starting at level 36 while holding an Ice Heal or a Burn Heal.
Noxial has two forms, the choice of which depends on the item Phlask was holding when it evolved. Holding an Ice Heal will cause Phlask to evolve into the Exothermic form (red), while holding a Burn Heal will cause Phlask to evolve into the Endothermic form (blue).
The two forms have access to different form-exclusive moves that they can learn by leveling up. Exothermic Noxial can learn some Fire-type moves, while Endothermic Noxial can learn some Ice-type moves. Both forms have access to most of the same TM and Move Tutor moves, sans opposite offensive Fire/Ice moves.
Noxial's evolution into Fumighast depends on its form. Exothermic Noxial (red) must hold a Burn Heal, while Endothermic Noxial (blue) must hold an Ice Heal. This is the opposite item to the one used to evolve Phlask into that form of Noxial.
Once Noxial evolves into Fumighast, it is no longer capable of learning any form-exclusive moves it could learn as a Noxial, not even if it had learnt those moves previously.
Noxial have been discovered in both endothermic and exothermic forms. The more extreme the temperature, the more volatile the Noxial.
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.
Noxial is based on a haunted chemistry set.
Noxial's name is a combination of noxious and vial.