Nightmare Luffy was Luffy’s form after he was greatly powered up by one hundred shadows implanted into him.
With Luffy’s strong spirit, he managed to contain one hundred shadows within him when most normal people could only contain two or three. In this form, Luffy was several times bigger than his normal self, and his skin turned completely blue. He was roughly the same size as Moria, and his body was similar to Chopper in Heavy Point. Luffy also talked differently, losing his usual carefree tone and instead speaking with a grim seriousness.