Diablon

The U78-X/D "Diablon" was a prototype energy-charged weapon built by the Guardian Units of Nations and one of the final bosses in Shadow the Hedgehog. It took the form of a large red mecha that was piloted by the Commander (possibly remotely) and fought alongside Sonic the Hedgehog against Shadow the Hedgehog after the dark mission was completed in the levels GUN Fortress, Black Comet, and Final Haunt in story mode. Both Sonic the Hedgehog and Diablon were defeated by Shadow the Hedgehog by using the latter's weaknesses against it.

History
The Commander used Diablon to try and avenge those who died on the Space Colony ARK (Including Maria Robotnik and his family) by fighting alongside Sonic in the hopes of taking down Shadow, whom he viewed as responsible for the ARK's incident. The duo fought well, but they were both defeated at the Ultimate Life form's hands by exploiting Diablon's weaknesses.

GUN Fortress Boss (Pure Dark)
After destroying the three Mother Computers in G.U.N.'s last stronghold on the entire planet, Sonic and Diablon confront Shadow in one last attempt to stop him from fully realizing the Black Arms' ambitions. After the Commander blames Shadow for the ARK incident and vows to destroy him, Shadow simply smirks and states his intentions to bring order to humans, and to make them "pay with their lives" for their crimes.

After Shadow defeats the duo, his inner turmoil drives him insane, and he regresses into the omnicidal living weapon of mass destruction that Professor Gerald Robotnik reprogrammed him to be. He vows to use the Chaos Emeralds to destroy the Earth, and laughs maniacally as he stands over a beaten Sonic.

Black Comet (Semi-Dark)
Sonic and Diablon confront Shadow after he fights off G.U.N.'s counterattack on the Black Comet. After the Commander blames Shadow for the ARK incident and vows to destroy him, Shadow simply smirks and states his intentions to bring order to humans, and to make them "pay with their lives" for their crimes.

After the duo fail to compensate for the counterattack's failure, Shadow becomes Black Doom's second-in-command, believing Black Doom's views that humans are evil. He vows to use the Chaos Emeralds to conquer Earth and turn it into a dark empire, before laughing maniacally as he stands over a beaten Sonic.

Final Haunt (Pure Hero, Dark)
Shadow is confronted by Sonic and Diablon after turning on the Black Comet's Defensive Energy Shield, with Sonic questioning Shadow on why he would turn against Earth at the last minute and help the Black Arms when their defeat was seemingly imminent. Shadow then replies that he was not necessarily "siding" with the Black Arms, but his motivation for doing so was to oppose Sonic himself and prove his superiority by defeating both him and the Black Arms.

After battling the duo and emerging victorious, Shadow stands over a defeated Sonic, who in turn muses that he "didn't think he had it in him". With the Chaos Emeralds in tow, Shadow proclaims himself to be the strongest hedgehog in the world, and laughs triumphantly.

Powers and Abilities
Diablon was fitted with an electromagnetic barrier that, when activated, deflected most projectiles, including firearm shots, laser blasts and even physical blows. In terms of weaponry, it had a laser cannon secured on its forehead that could fire long-ranged laser beams in any direction, and also had a chargeable Antimatter Cannon which could destroy easily damaged objects in a large dome-shaped radius around Diablon after charging up enough energy. Apart from that, the only other abilities Diablon was seen to use were hovering and moving around in the air by using its jet boosters, and swiping at enemies with its hands. It's stated in game that Diablon was a prototype and possibly not yet fully equiped.

Weaknesses
Though Diablon's electromagnetic barrier could deflect various assa, when the barrier was shut down so Diablon itself could strike its foe, it was considered "nothing but mere scrap iron" left wide open to firearm and physical attacks. The head appeared to be its weak-point, as Black Doom ordered Shadow to strike it. It was also vulnerable to Chaos Blast attacks, even with its barrier in place. However, at the beginning of the fight, the head is too high to reach by simply jumping, so players must try and use the Homing Attack when Sonic jumps to propel Shadow high enough to hit the robot's head.

Battle
During the boss battle Sonic and Diablon, Diablon is the actual boss; Sonic cannot be damaged but can be knocked down. Shadow's guide during this battle is Doom's Eye.

You must use the Homing Attack at Sonic while Sonic is jumping to bounce off Sonic to reach and attack Diablon for the first quarter of the fight. Once a certain amount of Diablon's health is depleted, his repulsers will fail, allowing the player to attack without having to hit Sonic first. He will use more attacks and have a shield around him. He is vulnerable as he attacks, so hit Diablon while he is attacking. When he says "Charge complete," get him to start using it and once he says "Antimatter cannon. Fire!", run out of the line representing the range of it. Watch out for Sonic, he will try to use homing attack on Shadow. Jump to reflect his homing attack. When your dark gauge is full, use it as many times as you can, because Chaos Blast can pierce Diablon's shield, or you can grab a weapon when your dark gauge is full and exploit both your invincibility and unlimited ammunition.

Help
Shadow can obtain a pistol around the room if you destroy the correct box. This is not necessary, but can be useful if your Dark Gauge is full which will give unlimited ammo. You can deal more damage this way than using Chaos Blast. This cannot pierce through his shield, so just fire as fast as you can when it's down.

Trivia

 * In all three endings in which Diablon was defeated, it was seen disabled but intact.
 * Even though the boss title is called "Sonic & Diablon", the real boss is Diablon, since Sonic can't be damaged, but is an obstacle; in fact, Sonic serves as an assistance to defeat Diablon.
 * Interestingly when you try to shoot at Sonic he will dodge left and right the same way as Shadow when dodging left or right while strafing.
 * Strangely, when you try to let Sonic chase you by going circles at Diablon, he will continue to run around the arena after the player stops moving and he will not harm the player during this process.
 * Although the Commander shouts "Antimatter Cannon, fire!" before he releases a blast of some sort of energy, the cannon is not likely to fire actual antimatter.
 * Antimatter destroys everything it comes in contact with unless it's in the form of charged particles. Even so, the blast would still disintegrate everything it touched that was not already made of antimatter, including Shadow, Sonic, and the Diablon itself.
 * Additionally, antimatter costs millions of dollars just to create milligrams of it and it is unlikely that even G.U.N. has enough money to create enough for one attack, let alone multiple.
 * However it must be taken into consideration that antimatter may not work in the same way in the Sonic Universe as in the Real World as there exists other energies unique to the Sonic Universe.
 * Sometimes near the end of the battle, Sonic will 'hide' under Diablon and not attack any longer. This may explain why he is seen lying barely underneath it in the ending cutscenes.
 * It could be possible that the G.U.N Fortress Boss vs Diablon was canon to the Last Story.
 * Diablon's name is derived from the Spanish word for devil, diablo. Coincidentally, whenever Diablon takes a swipe at Shadow, the commander says "Hold still, you devil!"
 * This is the second final boss in the series to be comprised of an upper torso and arms, as the first was Big Arm originally from Sonic the Hedgehog 3.