Nightmare Nega-Storm is the result of a fusion between Nega-Storm and Mecha Storm. This fusion is similar to how Android 13 turned into Super 13, with Mecha Storm giving Nega-Storm massive amounts of energy.
Nightmare Nega-Storm almost looks nothing like Nega Storm. Instead of upward curving quills, he gains Sonic's and Shadow's quills. He also gains Knuckles' hair and wings, although they can't be used for flight. His jacket is longer, his shirt is blue, his gloves change, and his shoes turn blue. He is extremely violent and powerful.
Nightmare Nega-Storm's first appearance was when Perfect Storm beat Mecha Storm. The robot gave his remained energy to Nega-Storm, trigerring the transformation. Nightmare Nega-Storm easily overpowered Perfect Storm, and even when Storm transformed into Perfect Dark Storm he couldn't hurt Nega-Storm. Seeing no other way to defeat him, Storm summons Chaos and fuses with him, resulting in Chaos Storm. Storm's power boost was enough to match Nightmare Nega-Storm. Chaos Storm then tried to access the Perfect Form, resulting in Semi-Perfect Chaos Storm. Semi-Perfect Chaos Storm defeated Nightmare Nega-Storm, causing Nega-Storm and Mecha Storm to de-fuse.
"Are you ready to meet your maker, Storm?"
"You are most weak. I am invincible!"
"Nooooooooooo! How could I have been beaten?! I am perfect!"
"You call that a challenge?!" - S Rank
"Too easy." - A Rank
"That was alright." - B Rank
"Shouldn't have played around." - C Rank
"I don't believe this." - D Rank
"Disgusting." - E Rank