"You received the light. This is... your mission!"
- ―Mephisto's last words, as Nexus ends both clone and template.
- Giant of Darkness (闇の巨人 Yami no Kyojin)
- Black Devil (黒い悪魔 Kuroi Akuma)
In Ultraman F, the comparison is further alluded through Soso and Utsuutsu, who became the aforementioned Giants of Darkness after selling their souls to Dark Zagi.
Mephisto's design motif is that of the Grim Reaper.
"Power is the real truth. Indeed that is true. Farewell, human."
- ―Mizorogi after merging with Mephisto.
Dark Mephisto was created approximately a year before the series began. Having orchestrated a Beast Human attack in an abandoned facility, Mephisto encountered Mizorogi and quickly swayed the man to the dark side and engineered several events in behind the scenes, including the death of Riko Saida and her possession of Dark Faust.
After Faust's death, Mephisto made his move by trying to sway Kazuki Komon, and later Nagi Saijyo, to his side. During that occasion, he fought Ultraman Nexus by himself and had deployed two Space Beasts under his control, Nosferu and Galberos. As Jun Himeya deduced, Mizorogi/Mephisto's actual intention was to have fun toying with his victim's suffering and fight against the Ultraman himself. This was proven by forcing a boy to witness his sister's demise by the Night Raiders when under capture of Nosferu, although both ended up rescued in the end and mind-wiped by the Memory Police. Following Golgolem's destruction, Mizorogi had Kutuura fed on its victims within six points in Shinjuku as a silent way of delivering his message to the Night Raiders.
"In power...I have the ability to rule the future. You, on the other hand, are wounded. There's no way you can beat me!"
- ―Dark Mephisto, episode 24
On the seventh point of Kutuura's attack, Nexus was dragged towards the Land of Death to fight against said monster while Mizorogi kidnapped Himeya's senior Megumi Sakuta and forced her to watch Nexus' defeat. Through all seven points of Kutuura's attack, he also purposely lured the Night Raiders to fight him while planning to harvest the captured Ultra' light and adding it to his strength. As it seemed that all hope is lost, followed by a weakened Nexus struggling to fight against Mephisto, the Night Raiders used the last of their Ultimate Vanisher on Nexus, restoring his strength and turn the tables of the battle. The dark giant went surprised as Nexus managed to perform beyond his calculations and their battle ended with both sides vanished.
Mizorogi survived but the stresses from previous battle caused his memories to be erased. He was later captured during Banpira's reign of terror and forced into interrogation for the TLT members to uncover the secret of Mephisto's power. When his memories resurfaced, he escaped custody and took Memory Police member Mizuo as his hostage. When Hiroyuki Misawa unveiled as the new dark giant Dark Mephisto (Zwei) in Mizorogi's place, the latter's sheer willpower allows him to transform into Mephisto for the last time and held his clone for Nexus to finish the both of them off. Mizorogi revealed to Nagi that the mastermind is someone who has slipped into their ranks before dying in her arms.
Ultraman Ginga S The Movie: Showdown! The 10 Ultra Warriors!
When Nexus, Cosmos, Max, Mebius, Zero, and Ginga Victory arrived at the first floor of Etelgar's Space-time Castle an Eteldummy (エタルダミー Etarudami) of Dark Mephisto was created to stop them from reaching Etelgar. He fought against Nexus while the others left to find to find Etelgar. The Eteldummy was destroyed by Nexus' Junis form with his Over-Ray Schtrom.
- Height: 50 m
- Weight: 50,000 t
- Eyes: Mephisto's eyes can clearly see in the dark of night or the Dark Field. Unlike most Ultra Eyes his are black.
- Energy Core: Mephisto's Color Timer-like anatomy, as a dark Ultra, is black and flashes red when he is injured.
- Armed Mephisto (アームドメフィスト Āmudo Mefisuto): The bracers on his arm, they work in a similar way to Nexus' Armed Nexus.
- Ultra Armor: Mephisto's skin is resistant to fire and laser beams like any Ultra Armor.
Mizorogi holds the Darkevolver in front of him and pulls it open. The central stone shines a purple light that covers his body. The transformation scene is an image of Mizorogi's face covered in purple light, the image cracks and Mephisto pops out with a purple light behind him. After escaping from the dark influence, Mizorogi transforms into Mephisto by absorbing light energy around him and using willpower to transform. This time the transformation scene is an image of Mizorogi's face covered in golden light, the image cracks and Mephisto pops out with a purple and yellow light behind him.
- Mental Abilities: Mephisto can play with people's minds at his desire and speak through the weak minded. He can also show people hallucinations. He can also make humans experience the memories and sight of others. It is also shown that Mizorogi can see and hear what happens inside a Meta Field even when he's outside.
- Teleportation: Mephisto can turn invisible and teleport away, like Faust.
- Mephisto Claw (メフィストクロー Mefisuto Kurō): Mephisto has the Armed Mephisto, devices similar to Nexus' Armed Nexus, on his arms. The one on the right is modified to transform into a large dual claw on his right wrist that can stab and slash into enemies and launch a variety of energy attacks. It is also how he erects his Dark Field in Ultra Warrior form.
- Dark Shift Wave (ダークシフトウェーブ Dāku Shifuto Uēbu): Similar to Dark Faust's Dark Field, Mephisto can create it in human form, inside the phase he can create a dark energy barrier at will around himself or his allies when in human form. His Dark Field also has a green tinge to it when forming. He can also use the Mephisto Claw and inject it to the ground to form it. Unlike Faust, Mephisto has generated his Dark Field without Nexus first creating his.
- Mephisto Shot (メフィストショット Mefisuto Shotto): Fires green energy blasts from the Mephisto Claw.
- Mephisto Hyper Shot (ハイパーメフィストショット Haipā Mefisuto Shotto): Fires more powerful shots, they are red in color.
- Arm Blade: In an Ultraman Festival stage show, Dark Mephisto wielded a giant arm blade. The prop is modified from Giga Berserke's back blade.
- Dark-Ray Schtrom (ダークレイ・シュトローム Dākurei Shutorōmu): Mephisto can emit a + style purple and black energy beam from his left hand, comparable to Nexus's Over-Ray Schtrom.
- Dark Ray Feather (ダークレイフェザー Dāku Rei Fezā): Mephisto can launch dark arc shaped bursts energy exactly like Nexus's Particle Feather.
- Dark Grenade (ダークグレネード Dākugurenēdo): Mephisto can fire a strong explosive light bullet from his right hand. Can be used without the Mephisto Claw. Used to take down Nexus after he used Board Ray Feathers.
- Dark Ray Cluster (ダークレイクラスター Dāku Rei Kurasutā): Mephisto has his own version of the Dark Cluster attack, but his energy balls track the target where ever it goes.
- Beast Humans (ビーストヒューマン Bīsuto Hyūman): Mephisto can create or at least control beast cells, upon injecting them in a person they can be shown illusions so the beast can feed on negative emotions and eventually overtake the human. When placed in a dead person they became puppets for him to control. The Dunamist's Blast Shot has a special blast to expel and destroy Beast cells safely.
- Dark Defensor (ダークディフェンサー Dāku Difensā): Mephisto can erect a barrier with one hand or both, resembling a purple pool of water. He can also perform this in human form.
Dark Mephisto appears in this game as one of the Ultra enemies on Nexus Mode and later a playable character. To obtain him, the player has to defeat him on Nexus Mode.
When an alternate Dark Zagi appeared, Soso and Utsuutsu wish to become kings of the new world and in turn transformed into Giants of Darkness. The siblings became Dark Mephisto and Dark Faust respectively as they lay waste to Osaka, especially the Shinsekai district. When Akiko Fuji challenged them as Giant Soldier F, Mephisto summoned Jamila as their aid but said monster was killed by Ide via Mars 133. The two engaged in the battle against Fuji but neither of them were proficient fighters (due to their aforementioned frail physicality). Zagi joined them and managed to weaken F but once all three of them open fire on Fuji, her Mefilas Bots were radiated and in turn transformed her into Ultraman F. The female Ultra countered their attacks by killing Faust with the Specium Ray and wounded Mephisto with the M87 Ray. After Zagi's death, Soso tried to commit suicide by detonating the Earth with him but Utsuutsu's spirit prevented him from doing so. Soso eventually joined his sister to the afterlife dissolving into a mass of darkness.
- It is unknown which of Zagi's giants was created first but certain evidence and dialogue implies that Mephisto was created first.
- Mephisto is the first male evil Ultra Warrior to have a shown transformation item, and is also the first to have said device being sold as a toy.
- It is deliberately ambiguous how much of the actual giant Mephisto was left after his defeat at Himeya's hands. Mizorogi's much more subdued and peaceful nature after his return implied the giant was dead. However, his powers, voice, and mannerisms all return after Mizorogi regains his memories, implying Mephisto also chose to redeem himself.
- Mephisto is the second Ultra Warrior to be redeemed from darkness, the first being Ultraman Tiga. However, his redemption is on-screen.
- In the original planning, a different voice actor was chosen for Dark Mephisto before Takeshi Yagi decided to employ Mitsutoshi into the role instead.
- Dark Mephisto's lacks a Color Timer noise in the series, hence for his appearance in Ultraman Nexus video game, he received a blinking noise that was recycled from Ultraman Neos.