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The "power" of the Flame of Solaris, an energy-based being. He has been sealed away inside of Elise's body by her father. Iblis is a gigantic monster of mass destruction; he is made of lava, and is practically immortal. Iblis spreads the Flames of Disaster throughout the world, bringing about the ruination of the planet. He was supposedly released by Sonic at some time in the near future, making it Silver's quest to find Sonic and stop him from unleashing Iblis, and destroying the world.
- Sonic's story: (3rd boss [Phase 2]) After the Flame Core level, Rouge finds a Chaos Emerald, but Shadow warns her not to touch it after it's too late and Iblis rose up from the flames and lava the second she touched it. Sonic now must defeat Iblis by standing on platforms with orbs that you can make light up, making Iblis coming to head-butt the platform that Sonic's on so he will fall off into the lava, but instead Iblis crashes into the foothold, which is Sonic's chance to perform a homing attack on Iblis.
- Silver's story: (1st and Final boss [Phases 1 and 3]) After the Crisis City level and Flame Core level, Iblis rises up, ready to attack Silver and Blaze. Silver must now defeat Iblis by launching lava rocks and debris at Iblis's armored head.
- Shadow's story: (2nd boss [Phase 2]) Its the same boss fight as Sonic's, but is fighting with Shadow instead.
A giant robot dog-like creature created by Dr. Eggman. Has a long fiery horn on the top of his head, and when Sonic or Shadow climbs up to his horn and hangs onto it, they can control the robot dog and make it run into walls to get hurt.
- Sonic's story: (1st boss) Once headed to the desert with Elise, Sonic gets interrupted by Egg-Cerberus and must defeat it by grinding up rails on the creature's tail and back to get to its horn, which Sonic must hang on to in order to take control of the creature and make it run into walls to get hurt.
- Shadow's story: (1st boss) After the White Acropolis level, Shadow and Rouge get interrupted on their SOS mission by Egg-Cerberus and must defeat it by grinding up rails on the creature's tail and back to get to its horn, which Shadow must hang on to in order to take control of the creature and make it run into walls to damage it.
Egg-Genesis is the vehicle Dr. Eggman is riding in during the boss Egg-Genesis. It's a flying vehicle with four ends coming out of it, which is where missiles are shot at you and enemies get created coming out of the ends. (Note: may be named after Sega-Genesis)
- Sonic's story: (4th boss) After the Radical Train level, Elise gets captured once again by Dr. Eggman, and says she'd rather die than be Eggman's prisoner, so she goes up to the end of Eggman's vehicle, and leans back and falls off the vehicle. She would die, but luckily, Sonic catches her. Eggman doesn't believe Sonic can get away THAT easily, so he decides to attack Sonic with his vehicle Egg-Genesis. Sonic must defeat Dr. Eggman by waiting for Dr. Eggman to create paths of enemies while avoiding missiles and laser blasts, then homing attack up the paths of enemies to land on one of the ends of the vehicle. Then, Sonic must walk up the end to the very top center of the vehicle, which he can homing attack and/or bounce attack the top center of the vehicle after avoiding missiles to make the boss lose health. (Note: an easier method entails running under the Egg-Genesis when it lowers, using bounce attacks to gain height, followed by 3 homing attacks on the orange core laser.) Either way, after the Egg-Genesis loses all of its health, it crashes down to the ground where Sonic is and Sonic has to avoid it and make sure it doesn't crash into him.
- Silver's story: (3rd boss) After the White Acropolis level, once Silver and Blaze sneak into Dr. Eggman's base, they find Dr. Eggman there testing out his new vehicle, the Egg-Genesis. Silver must defeat Dr. Eggman by lifting up the enemies after stunning them and missiles and crates with his physic powers and launching them at Eggman to hurt him while at the same time is also avoiding the other missiles and enemies, not to mention the laser blasts. After the Egg-Genesis loses all of its health, it crashes down to the ground where Silver and Blaze are and they have to avoid it and make sure it doesn't crash into them.
Egg-Wyvern is not only the dragon-type vehicle that Dr. Eggman rides in a boss, but it's also the name of the Final boss in Sonic's story. Has a long fiery horn on the top of the vehicle's head, and when Sonic climbs up to his horn and hangs onto it, he can take partial control the robot dragon vehicle and make it fly into pieces of debris falling from Dr. Eggman's Egg Carrier to damage it, similar to Egg-Cerberus. Unlike the first boss fight, however, the Wyvern resist control from the start, making ramming it into the falling debris more difficult. Further, after taking away half of its life the protective canopy covering Eggman falls away. During this time, instead of controlling it, the means of attack is similar to Egg-Genesis in that Eggman releases his gun-arms and Sonic must homing attack to them, and then hit Eggman. The final hit is the same as the first half. Eggman once again tries to ram, and Sonic must homing attack him, destroying the Wyvern, and ending the fight, as well as Sonics specific segment of the game. This boss is unique to Sonics story. The Egg-Wyvern shares similarities to Bahamut SIN from Final Fantasy VII: Advent Children.
- Sonic's story: (Final boss) Sonic runs into Dr. Eggman once again after the Aquatic Base level and Sonic must defeat him by dodging lasers and laser blasts until Dr. Eggman comes to charge Sonic, which is when Sonic can homing attack the horn on the dragon vehicle's head to take control of the boss and make it fly into pieces of debris falling from Dr. Eggman's Egg Carrier ship to get hurt.
Sonic VS. Silver
In Sonic's story, you have to face Silver for the 2nd boss after the Dusty Desert level by performing an attack on Silver while he's busy picking up things to throw at Sonic. Attacking him other wise results in Sonic being controlled and thrown against either the sky or a wall, either way resulting in damage. Also in Silver's story, you have to face Sonic for the 2nd boss after Dusty Desert by picking up crates, tables, and chairs and throwing them at Sonic with your physic powers while Sonic isn't homing attacking you.
Shadow VS. Silver
Silver fights Shadow after completing the level Radical Train for the 3rd boss (or Shadow fights Silver depending on which story you are in), because Silver is mad at him for halting Sonic's death (whom Silver and Blaze think are the Iblis Trigger, thus assuming it a good thing). Also, at first, Silver mistook Shadow for Mephiles.
In Shadow's story, you get to face Mephiles in his crystalline form as Shadow in the first part and Omega during the second part in Phase 1.
Mephiles also serves as Shadow's final boss in Phase 2. During this battle, Mephiles takes the form of a shadowy monster, and Shadow must build his Chaos Gauge by killing various enemies. Once it is full, activating it reveals Mephiles' true form, allowing Shadow to defeat him.
Solaris is the game's final boss and has two phases. Iblis has two forms: he first becomes a mindless enormous lava monster when released, but when he merges with Mephiles he becomes Solaris, a being that is able to control the vastness of time and space. He is fought in the games "final episode". After gathering the Chaos Emeralds, Elise kissed Sonic and he is brought back to life in his Super form. Extending his energy outwards, he transforms Shadow and Silver into Super Shadow and Super Silver, respectively. The three of them, Super Shadow, Super Silver, and the recently revived Super Sonic from the power of the Chaos Emeralds, go to finish the fight. Solaris is destroyed by the three hedgehogs' united power and they each return to their own respective times. The game hints the three will destroy him in each of the separate times, but the actual battle makes it seem more likely that the fight him in the present. As this "present" is a temporal void comprised of multiple destroyed time periods, the past present and future are all represented here anyways. Following his destruction, the final cutscene is shown.