Take a look at the sentence again:
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 is a transition that is interrupting the flow of the sentence. When a transition interrupts the flow of a sentence, it requires a comma both in front and behind it.
Go back to the sentence to try again.