Review the item:
Chet decided to get a huge Chinese dragon tattooed across his shoulder blades
once the stabbing needle pierced his skin,
however, Chet quickly changed his mind, opting to pierce an ear instead.
However, a conjunctive adverb, is interrupting the flow of the sentence. When a conjunctive adverb interrupts the flow of a sentence, it requires a comma both in front and behind it.