"Don't blow your wig, Dice. If those two little finks try anything...I'll be waiting!!!"
—The Devil to King Dice
The Devil glares at Cuphead and Mugman, and his pupil turns into image of cup being shattered and its shards being sliced into nothing. This intro is unique, because unlike in every other intro, Cuphead and Mugman are terrified instead of getting prepared for battle.
"You broke our deal...now it's my turn to break you!!!"
—The Devil, Phase 1 death screen
The Devil's first phase involves him attacking with both his trident and his transformations.
By chanting an incantation and swinging his trident, he can create one of three projectile attacks:
- Creating four crystal balls that bounce around the arena erratically. One of the four can be parried.
- Creating five fireballs moving left to right and back; four blue fireballs revolve around one pink one, which can be parried.
- Creating six fireballs fixed in a hexagon formation, which home in on the players. One of the six can be parried.
He also uses three transformations, each with its own attack:
- Spider: The Devil grows four extra arms and detaches his head as his head grows four extra eyes, two smaller horns and four small legs to resemble a spider. In this transformation, he primarily attacks by slamming down three times before reverting to his original form.
- Ram: His head becomes a ram and he extends his arms from both sides to crush the players in the middle. This attack can be tricky, as players have to jump at the precise moment to not get hit. Alternatively, the player can crouch to avoid this attack.
- Dragon: His head becomes a serpentine dragon and extends to either side as a means to corner the players so they can be hit by his minions.
Throughout the phase, his purple demon minions hiding behind his throne will come from either side to damage the players. These minions can be disposed of by simply shooting at them.
After taking enough hits, The Devil will pull his skeleton out of his skin and escape through a small hole in the arena's center. The players must follow him into the hole or be damaged by advancing walls of fire on either side of the screen.
"Anyone who opposes me will be destroyed!"
—The Devil, Final phase death screen
After escaping through the hole, the final battle commences as the Devil grows into a giant and players fight him on five floating platforms. In this phase, players have to shoot at his eyes while dodging his two attacks:
- By joining his two eyes into one, he summons a fiery axe that moves in a spiral motion that will damage the players if they run into it.
- A bat bomb is created from one eye and spawned through either the left and right ears, depending on the eye he spawns them in. If the players fail to neutralize the bomb by parrying it, the bomb will explode covering a wide radius.
Throughout the phase, flaming poker chips fall from the sky and land on random platforms, damaging the players standing beneath them.
After taking enough hits, the Devil will become frustrated and the platforms will be reduced to three which increases the chance of poker chips falling onto the players. The Devil will no longer fight Cuphead and Mugman directly; Instead, he will summon his minions to damage the players in one of two ways:
- He will summon two fat purple winged demons from both side to shoot skulls at them. Occasionally, pink skulls appear that can be parried. These demons can be shot down when enough damage is dealt to them.
- Above his head is a group of small, blue-winged demons that home in on the players. These demons can also be dispatched fairly easily by shooting at them.
After taking enough hits, the Devil will go into a physical and mental breakdown, losing his teeth and crying. In this final sub-phase, the Devil is reduced to using the poker chips and his tears to damage the players.
Only one platform remains in the center as players have to dodge the falling poker chip and dash back to the platform while avoiding his tears. Some of the tears can be parried.
After the Devil takes enough damage, he is beaten for good.
- The Devil's design is based on the devil from "Hell's Bells", a short from Silly Symphonies.
- There's an unused design of the Devil shown in the 2015 E3 trailer that is a vampire-like bat which fires other bats to attack.
- The unused design seems based on the Nightmare from the game 123 Slaughter Me Street.
- The Devil’s First Phase form is similar to Loki and Emperor Sardius, respectively the final bosses from Capcom’s Ghouls 'n' Ghosts and Super Ghouls 'n' Ghosts.
- The animation for his Final Phase may have been inspired by the lightings on Chernobog from Fantasia's Night on Bald Mountain.
- The screen when refusing to give the contracts to the Devil had animated variant used in E3 2017 trailer, but never used in the final version. This might have hinted, that the game supposed to have animated cutscenes.
- The Devil's four crystal balls attack is a close match and a possible reference to one of Crystal Man's attacks from Mega Man 5.
- At the end of the "Dragon" attack in Phase 1 of the Devil's boss fight, there is a lingering hit-box in the bottom left-hand corner of the screen. This means that if the player stays in the corner after the attack has ended, they can be hit by a seemingly invisible object and lose life.
- Due to a bug, it is possible to deal damage to the Devil by shooting a hurt-box that appears off-screen to the left of the stage. This can be exploited to defeat The Devil's final phase very easily.
- If you agree to give the contracts to the devil, it will lead you to the "bad ending."
- The Devil, like King Dice, cannot be fought in Simple difficulty; only on Regular and Expert.
- As you keep on attacking The Devil in phase 2, he appears to age back into a child, as seen the second time when he starts crying.
- In the games code, Phase 2 is labeled as Phase 3, meaning that there is a phase missing.
- Phase 2 could possibly be the unused design.