Kage Kage no Mi/Nightmare Luffy

Nightmare Luffy (ナイトメア•ルフィ) was Monkey D. Luffy's form after he was greatly powered up by one hundred shadows implanted into him.

Personality
In this form, Luffy talked differently, losing his usual carefree tone and instead speaking with a grim seriousness.

Appearance
In this form, Luffy was several times bigger than his normal self, far more muscular and his skin turned completely blue. He was roughly the same size as Moriah and his body was similar to Chopper in Heavy Point. Nightmare Luffy was big enough to carry a normal-sized person by the waist in one hand. He carried a sword strapped to his back, and stopped wearing his vest for some reason, but returned to wearing it when he returned to normal.

Abilities and Powers
As Luffy possesses the Gomu Gomu no Mi, he is rubber and can stretch his fists to increase his range and attack momentum. Being further augmented by the 100 shadows, all of his normal techniques are further increased, enough to topple even Oars. With Luffy's strong spirit, he was able to contain one hundred shadows within him when most normal people could only contain two or three.

While in this form, Luffy was capable of using the combined strength and skills of the shadows within him. With this strength, Luffy was even capable of stopping a giant Gomu Gomu no Rifle attack delivered by Oars with one hand and of delivering massive attacks onto the zombie behemoth. Because one of the shadows belonged to a swordsman, Luffy also gained the ability of fighting with swords, something he never knew before. He also appears to be able to move at an incredible speed, having safely snatched up both Nami and Usopp away from a rampaging Oars and it was only when Luffy made his presence known did Oars and Moriah learned about it.

This form however only lasted for ten minutes. After that, all of the shadows within Luffy returned to their original owners.
 * Gomu Gomu no Pistol (ゴムゴムの銃（ピストル）): Luffy's signature attack, a straight forward punch augmented by his elasticity stretching powers. As Nightmare Luffy, its strength is further augmented. In the game Unlimited Cruise, Nightmare Luffy adds momentum to this attack by spinning his arm around before "firing" the fist.
 * Gomu Gomu no Storm (ゴムゴムの暴風雨（ストーム）): One of Luffy's more powerful attacks, by inflating himself and then twisting his body, and using the momentum of untwisting and exhaling to release a massive barrage of fists upwards. As Nightmare Luffy, he uses an alternate version of the normal technique of the maneuver by twisting himself and them shoots himself sideways and hits Oars with a barrage of fists, all of which are far more powerful than his normal form. In the game Unlimited Cruise, Nightmare Luffy performs yet another alternative version by jumping upwards and launching the fists downwards.

Thriller Bark Arc
After meeting the Thriller Bark Victim's Association, Luffy promised to help them get their shadows back from Moriah. In order to help him, the association implanted one hundred shadows that they have gathered over the years into the Straw Hat captain, which resulted in Luffy transforming into "Nightmare Luffy". He first tested out his powers by utilizing expert swordsmanship to slice apart tree stumps, something that the he would never be able to do in his normal form. He then ran off to face Moriah, only to be informed that he was going in the wrong direction.

Nightmare Luffy eventually made it to the battlefield, where his crew was fighting Oars and Moriah. He rescued both Nami and Usopp from being killed by the giant, and then attacked. Oars tried to use his massive size to overwhelm his shadow's originator, but Luffy easily caught the super-sized fist, and then launched his own attacks, all of which were heavily augmented by the shadows. Within ten minutes, Luffy launched his Gomu Gomu no Storm technique, knocking down Oars and Moriah fell unconscious from the attack. With that, the one hundred shadows left Luffy's body, reverting him back to normal.

Early Concepts and Other Media
Early concepts of Nightmare Luffy were revealed in One Piece Green: Secret Pieces, one of them was a "polite and serious" Luffy. Another somewhat resembled Freddy Krueger from the "Nightmare on Elm Street" franchise. Also, none of them seem to be as bulky looking as the final version.

In the game Unlimited Cruise, Luffy, in this form, has also displayed other abilities to compliment those seen from both anime and manga. Such abilities include being able to breath three fireballs horizontally after drinking some liquid from a calabash, and shooting out multiple flaming arrows at a considerable range with a longbow. The game also made more elaboration on the skills he demonstrated in the manga and anime, such as using a nodachi by jumping up and slashing downwards at his opponents, and his high-speed movements are similar to the Soru technique. His Gomu Gomu no Pistol involves spinning his fist in a circle before launching it to increase momentum, and his Gomu Gomu no Storm has him jumping upwards while inhaling and twisting before launching the barrage downwards.

In the game Gigant Battle, Nightmare Luffy appears as a support character. He will appear when the main player calls on him to attack, and then leaves. However, if the main fighter is Luffy, then he will instead transform into Nightmare Luffy to attack before returning to normal.