I would just introduce the Goblin getup as Harry’s Halloween costume. Maybe Midtown High or ESU is holding a costume contest, and Harry spends a lot of money on a really impressive cloaked green goblin costume. Norman and Harry have had a really tumultuous relationship, and perhaps something really awful happens between them on Halloween. For example, Harry defies Norman, and Norman (at his wits end) becomes physically abusive. Harry wearing this costume on Halloween is Norman’s first “encounter” with the Goblin. Shortly after taking a serum or experimenting with chemicals at Oscorp, Norman begins having these hallucinations of the same cloaked goblin figure berating him or coaxing him into acting on the violent thoughts that he’s been having lately. When Norman finally submits to these destructive thoughts, he wears the goblin mask and remnants of the torn and muddied purple cloak over a black (or dark green) bodysuit that was designed to be worn with his glider. The costume not only becomes a physical manifestation of Norman’s insanity and a physical representation of the evil that lies within all of us, but also symbolizes Norman and Harry’s complex relationship. Since Harry was seen wearing the mask first, you could even pull a TSSM, and have the audience (and Peter) believe for a while that the Green Goblin is really Harry instead of Norman.