Flame Core

Flame Core is an Action Stage in Sonic the Hedgehog. Like Crisis City, the Action Stage occurs at least 200 years in the future, in a fire-filled timeline caused by Iblis' release. In Sonic and Shadow's stories, Flame Core hides a Chaos Emerald. This location is inhabited by Iblis type monsters and Mephiles monsters. Iblis is the boss to face afterwards. Mephiles is also fought here in Shadow's story, after Tropical Jungle.

Sonic
After surviving Crisis City, Sonic The Hedgehog follows Shadow and Rouge to the depths of Flame Core, a volcanic wasteland that pulsates with deadly lava ﬂows and eruptions. Sonic must negotiate the rivers of molten earth to find the heart of the volcano, where only the powerful Knuckles can help the hedgehog light a path through the darkness. As Sonic stabs deeper to the core, and after he engages in a fearsome showdown with one of Iblis’s many forms, he must ﬁgure out how to undo the time wrinkle made by Dr. Eggman’s terrible machine and return to the present to help Princess Elise. Hopefully, he can withstand the withering heat of the volcano long enough to connect with Shadow The Hedgehog, who may have the secret of bending time and space.

Sonic starts off outside the volcano then travels inside it. Sonic then fights Iblis Phase 2. After that he does a "Cross-Chaos Control" with Shadow to escape to the present.

Shadow
Before he can return to his own time, Shadow must venture deep into the Flame Core and confront Iblis. Reaching the living ﬂame is no easy task, as Shadow must ﬁght through legions of ﬁre monsters that obey Iblis’s every vicious whim. Not only must Shadow somehow defeat all of these minions, he must dive deep into the heart of an active volcano. Some of the lava tubes are bathed in darkness, so Shadow must defer to his partner Rouge the Bat to feel her way through the inky blackness and light a path to Shadow’s showdown with Iblis.

Shadow's version of the stage is slightly harder than Sonic's. He starts off outside the volcano, then goes inside. Shadow fights Iblis and Mephiles-type monsters. Afterwards he fights Iblis Phase 2. Then he "Cross-Chaos Controls" with Sonic, but then decides to stay behind and fight Mephiles. Shadow is later sent back to the present because he (along with E-123 Omega) follows Mephiles via a time warp.

Silver
Silver The Hedgehog and his friend Blaze have ﬁnally fought their way to Flame Core, the source of Iblis’s strength. The bubbling lava ﬂows and raging rivers of molten earth are deadly to the touch, so Silver must harness his Psychokinesis to Levitate over the ﬁres. Serpentine ﬁre monsters and wild phoenixes put up a mighty defense for their master, the living ﬂame. To succeed, Silver must push through the denizens of disaster and negotiate the blistering heat to locate Iblis’s lair. When he ﬁnally confronts Iblis, Silver has only minutes to defeat the monstrous force and alter his ruined future.

Silver starts off outside the volcano and goes inside it. Silver fights Iblis and Mephiles monsters. At the end of the stage he destroys a giant ball of flame, which is part of Iblis. Afterwards he goes on outside and fights Iblis Phase 3.

Omega
In the End of the World, Omega ventures here to find the green Emerald.

Sonic
In Sonic's Flame Core in Very Hard Mode, he actually starts out inside the volcano. He encounters Knuckles, like the original stage. However, when Sonic reaches the end of the volcano, there is a warp instead of a Goal Ring. This warp takes Sonic back to the beginning of the outside section of the stage, where he must get through more traps than the usual stage and reach the end of the outside section, where the Goal Ring resides.

Shadow
Shadow starts out inside the volcano as well, but at the end of the stage. After making a comment about going upward, Shadow comes out of the final shaft that he usually runs down to finish the stage. Rouge then takes over and she needs to find keys to move on.

Trivia

 * There is a glitch in Sonic's Flame Core stage where you can finish his stage as Knuckles, by climbing a cornered wall and gliding, however to see his stage finish screen the player must hit the last checkpoint.