The Dark Star Core (also known as Dark Fawful) is the fusion of the Dark Star and Fawful, and serves as the final boss for Mario and Luigi in Mario & Luigi: Bowser's Inside Story and Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey. It consists of Dark Fawful's remains and the Dark Star. In order for Mario and Luigi to attack the Dark Star Core, Bowser has to knock out Dark Bowser first. After that, the Dark Star Core will revive Dark Bowser. And after Dark Bowser has been revived, Bowser has to punch his stomach, forcing Dark Bowser to spit out the Dark Star Core. Bowser then uses Vacuum to suck up the Dark Star Core. Then Mario and Luigi can attack. Mario and Luigi must destroy Fawful so they can defeat the Dark Star once and for all. After a few turns, Dark Star will resurrect Fawful and leaves Bowser's stomach, forcing Bowser to spit the Dark Star Core into Dark Bowser, meaning the player must do the phase all over again. After defeating the Dark Star Core, Bowser can do the final punches to Dark Bowser. Then, Fawful explodes inside Bowser's stomach and expels everyone out of Bowser's body and negativing the effects of the Vacuum Mushroom. Then Bowser figures out that Mario, Luigi, Starlow, and the others were in his body. Enraged, Bowser battles Mario, Luigi and Starlow, however, the Mario Bros. managed to defeat Bowser. After the Dark Star Core exploded, it was gone forever.

The instant Dark Star Core gets defeated.
Trivia
- When one of Fawful's eyes are damaged, the Dark Star Core closes one of its eyes as well. This strongly implies that Fawful might be controlling the Dark Star and thus Dark Bowser, an idea further supported by the fact that Fawful tries to flee back into Dark Bowser's body when Bowser inhales him.
- The Dark Star Core is the second final boss to be fought inside Bowser's body. The first was Cackletta, Fawful's own master.
- The Dark Star Core is the third final boss of the Mario and Luigi series that has an attack that involves swinging an object around in circles. The other two were Cackletta's Soul and Elder Princess Shroob.
- The battle background is only seen in this battle even if the bros. are in an area where they had previously fought.
- Aditionally regardless to what area in Bowser's Body the player were in before the fight, the background in the cutscene as well as the fight are always the same.
- The Antenna attached to Dark Fawful's head with the Dark Star is similar to the antenna on his head in the final battle with him in Superstar Saga and in Partners in Time.
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Characters | Protagonists | Bowser • Luigi • Mario • Starlow |
Antagonists | Dark Star • Fawful • Midbus | |
Other Characters | Blue Paratroopas • Bowser Memory Neuron • Bowser Memory P • Broggy • Broque Madame • Broque Monsieur • Chakron • Corporal Paraplonk • Doctor Toadley • Doctor Toadley's intern • Fawful • Giant Sockop • Jerry • Kamek • Kuribo • Kuzzle • Monty Bros. • PR Toad • Princess Lipid • Princess Peach • Private Goomp • Sergeant Guy • Toadbert • Toadsworth • Yikk | |
Species | Bee • Blitty • Bob-omb • Brock • Emoglobin • Evoglobin • Goomba • Magikoopa • Nutsoglobin • Shy Guy • Totemglobin • Toad • Virus | |
Locations | Mushroom Kingdom | Badge Shop • Blubble Lake • Bowser Path • Bowser's Castle • Broggy's Shop • Broque Monsieur's Shop • Bumpsy Plains • Cavi Cape • Conference Hall • Dimble Wood • Fawful Theater • Gear Shop • Peach's Castle • Peach's Castle Garden • Plack Beach • Project K • Star Shop • Tea Kiosk • Toad Town • Toad Town Caves • Toad Town Mall • Toadles Boutique • Toadley Clinic • Tower of Yikk • Treasure Chamber • Tunnel |
Bowser's body | Airway • Arm Center • Challenge Node • Chest Station • Cholesteroad • Energy Hold • Flab Zone • Flame Pipe • Funny Bone • Gut Check • Joint Tower • Leg Outpost • Lumbar Nook • Memory Banks • Nerve Cluster • Nose Deck • Pipe Yard • Pump Works • Rump Command • The Gauntlet • Toad Square • Trash Pit | |
Enemies | Mushroom Kingdom | Alarm Bob-omb • Beehoss • Borp • Broggy • Broque Monsieur • Chain Chawful • Choomba • Chuboomba • Coalbit • Crawful • Dark Fawful Bomb • Dark Fawful Guy • Dark Mechawful • Dark Mechawful.5 • Dark Trashure • Drillbit Crab • Fawflant • Fawflopper • Fawfulcopter • Fawful Guy • Flifit • Glam Lakitu • Jailgoon • Lakitu King • Magifoofa • Mawful Mole • Mechawful • Metal Mawful Mole • Naplock • Nooz • Pendril • Scutlet • Sneed • Sniffle Thwomp • Sockop • Toothy • Treevil • Trashure |
Bowser's body | Airnapse • Air Cannon • Air Cheep • Bone Thwomp • Bowser Memory Boo • Bubble Blooper • Calorite • Dried Blooper • Elite Goombule • Goombule • Napse • Pep • Piranha Plorp • Protobatter • Pwallop • Spike Blop • Stonk • Tenderling | |
Bosses | Main Bosses | Alpha and Beta Kretin • Biffidus • Blizzard Midbus • Bowser • Bowser Memory ML • Chakron • Dark Bowser • Dark Fawful • Dark Satellmite • Dark Star • Dark Star Core • Durmite • Junker • Junker Can • Leaf Guy • Midbus • Scutlet • Sea Pipe Statue • Shroob • Snawful • Vacuum Helmet • Wiggler • Wisdurm |
Giant Bosses | Bowser's Castle • Fawful Express • Fawful Mountain • Super Peach's Castle • Tower of Yikk | |
X Bosses | Alpha and Beta Kretin X • Biffidus X • Bowser X • Bowser Memory MLX • Dark Satellmite X • Dark Star X • Durmite X • Junker X • Junker Can X • Wisdurm X | |
Techniques | Mario and Luigi | Falling Star • Fire Flower • Green Shell • Jump Helmet • Magic Window • Mighty Meteor • Snack Basket • Spin Pipe • Super Bouncer • Yoo Who Cannon |
Bowser | Bob-omb Blitz • Broggy Bonker • Goomba Storm • Koopa Corps • Magikoopa Mob • Shy Guy Squad | |
Items | Consumables | 1-Up Deluxe • 1-Up Mushroom • Cheese** • Fiery Drumstick • Heart Bean • Hot Drumstick • Max Mushroom • Max Nut • Max Syrup Jar • Mushroom • Nut • Power Bean • Refreshing Herb • Retry Clock • Special Bean • Star Candy • Super Cheese** • Super Mushroom • Super Nut • Supersyrup Jar* • Syrup Jar* • TNT Drumstick • Ultra Cheese** • Ultra Mushroom • Ultra Nut • Ultrasyrup Jar* |
Key Items | Banzai Bill • Birdley • Blitty • Blue Key • Green Key • Hammers • Miracle Cure • Mushroom Ball • Puzzle Sack • Red Key • Skeletone Formula • Star Cure • Star Panel • Stingler • Vacuum Block • Vacuum Shroom | |
Blocks | 99 Block • Attack Piece Block • Blue Shell Block • Command Block • Hidden Block • L Block • M Block • Rally Block • Save Block • Shop Block | |
Other | 5 Gold Coin • 10 Gold Coin • 50 Gold Coin • Blorbs • Boo-ray Machine • Cake • Chakroad • Chombone • Donut • Giga-carrot • Launch Fit • Liftoglobin • Meat • Mushroom Tea • Negative Ions • Nubbin • Scratch Card • Seesaw Fit • Squid Fit • Vegetable • Whirlwind | |
Miscellenous | Stats | Defense • Experience Point • Heart Point • Power • Rank • Special Point • Speed • 'Stache/Horn |
Status | Balloon Mario • Burn • DEF-Down • DEF-Up • Dizzy • Fire Mario • Fury • Level up • No Skills • Poison • POW-Down • POW-Up • Pump Mario • SPD-Down • SPD-Up • Shell Mario • Sick | |
Moves | Body Slam • Fire Breath • Flee • Jump • Shell Defense • Sliding Punch • Spike Ball • Trip | |
Other | Bros. Badge • Clothing • Gallery • Glitches • Mario & Luigi RPG: Sound Selection • Pre-release and unused content • Quotes • Shops • Staff • Star Menu | |
*-Exclusive in Mario & Luigi: Bowser's Inside Story • *-Exclusive in Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey |