How To evolve Baal'Thasar's starting weapon (Blacken Firmament) into Damnation in Vampire Survivors

Baal'Thasar is a special boss and unlockable character in Vampire Survivors introduced in the Legacy of the Bloodmoon update. The fight features massive HP pools and an extended phase where you must survive and attack the snake head/boss mechanics. Avoid Inverse Mode if you want the encounter to properly progress and end.