Mario & Luigi

Dark Star

A being of unknown origin, when the Dark Star was uncovered by Toad archaeologists it went on a rampage, requiring the efforts of the Star Sprites to render it dormant and seal it away once more. Should an evil being gain it for themselves, it was said that they would achieve such a great increase of power they’d plunge the world into darkness. Of course, this tale made it a target for evil beings wanting to conquer the world and plunge it into darkness. While Bowser would make an effort for it, it was Fawful that would successfully steal and reactivate it, successfully using it to increase his abilities. However, the Dark Star wasn’t willing to allow itself to continue being used, and would ultimately assimilate Fawful as it tried to conquer the world itself.

Dark Star - Physicals

Shatters barriers possibly made of bone after copying Bowser’s arms (source)
With copies of Bowser’s arms, shatters a cube of ice trapping it (source)
After creating Bowser’s shell on its back, body slams through floor possibly made of bone (source)
Regularly unharmed by the Mario Bros.’ attacks through a shield around it. It can be harmed by its own minions being hit back at it, ultimately damaging it enough to cause it to lose the shield, drop to the ground, and have its eyes disappear, allowing it to be harmed by regular attacks. It recovers soon after (source) (Not at full power due to Fawful having taken some of it for himself)
Seemingly unharmed while burning up a sticky substance trapping it (source)

Dark Star - Abilities

While dormant, is used to create a series of dark barriers that block the path to Peach’s castle. These were only destroyed by the Miracle Cure (source)
Consumes Bowser’s genetic information to copy aspects of him, though the information doesn’t settle well with it (source)
Copies Bowser’s ability to breathe fire, vaporizing a barrier inside his body (source)
Burns up a sticky substance it was trapped in (source)
Charges a large energy beam attack (source)
Makes a bomb (source)
Creates minions (source)
Multiplies into several copies while creating a thick fog around the whole area before spinning forwards to ram the Mario Bros. (source)
Emits light which turns the entire surroundings white, causing one or two copies of Mario and Luigi’s shadows to jump up and run around it. It shoots energy which damages the Mario Bro whose shadow it hits, which can be avoided through the plumber jumping, causing their shadow to do the same (source)
Turns into energy in order to enter and leave Bowser (source)
Does this to split up into four areas and escape the Mario Bros. (source)
While damaged enough to lose its shield and drop to the ground, shoots out a red beam while briefly floating (source)

Dark Star - Minions

Shoots energy beams (source)
When damaged enough, these minions become explosive. The Dark Star creates a bomb which they jump on, bring into the air, then drop between the Mario Bros. The minion can be hammered back to Dark Star, bursting on hitting and damaging it (source)

Dark Bowser - Proto Dark Bowser

Shatters a large pair of wooden doors with a punch (source) (Dark Star is not at full power due to Fawful having taken some of it for himself)
Destroys a large amount of rubble by punching it (source) (Dark Star is not at full power due to Fawful having taken some of it for himself)
Bursts apart to reform in a different location, and deactivates an energy barrier by punching it (source) (Dark Star is not at full power due to Fawful having taken some of it for himself)
Inhales in a manner to Bowser’s vacuum ability, pulling Luigi into him before spitting him out with enough force to send Mario skidding a notable distance back (source) (Dark Star is not at full power due to Fawful having taken some of it for himself)
Inhales an unconscious Peach into him (source) (Dark Star is not at full power due to Fawful having taken some of it for himself)
Inhales the Dark Star Core to reach full power, allowing him to fully turn into Dark Bowser (source) (Dark Star is not at full power due to Fawful having taken some of it for himself)

Dark Bowser - Complete Dark Bowser

Shatters a throne with a punch (source)
Jumps in midair as a spike ball, smashing through a stone ceiling (source)
Creates and throws a large boulder into the air (source)
Takes hits from Bowser (source)
Clashes fire breaths with Bowser (source)
Shatters a large boulder with a bolt of fire (source)
After having his mouth blocked by a boulder while charging a stream of fire, bursts with dark energy (source)
Creates a cage of dark energy around Bowser which rises into the air, with a lock that withstands a number of Bowser’s punches before breaking. Dark Bowser then charges a slow moving ball of dark energy before throwing it (source)
Throws a ball of shadow versions of Bowser’s minions, sending Bowser skidding back a great distance before it bursts, releasing Goombas, Koopas, Thwomps, Bob-ombs, and Bullet Bills Bowser has to circumvent getting back to Dark Bowser. When he does get back, Dark Bowser attacks him twice within his shell as a spike ball (source)
Contests inhaling the Dark Star Core with Bowser (source)
Covers the country in darkness, trapping many of its inhabitants inside dark bubbles (source)
After the Dark Star Core is defeated Dark Bowser is weakened, allowing Bowser to finish him off (source)

Dark Star Core

Takes hits from the Mario Bros. (source)
Lifts a Mario Bro in each hand before raising and lowering them, creating pulses of damaging stars at different levels (source)
Shoots a wide cone of energy (source)
Sends energy through its arms, creating a short wave on hitting the ground to damage Mario or Luigi (source)
Creates a spike which spins around before being run along the ground, leaving purple energy in its wake (source)
Has the giant Fawful head move along the ground as the Dark Star loops through the air, trying to ram Mario and Luigi into it (source)
Makes itself grow in size (source)
Heals Dark Bowser to full health from inside him, the process briefly making Dark Bowser larger (source)
Can regenerate its broken and destroyed parts (though not healing itself any HP) before floating out of Bowser and entering Dark Bowser (source)
After his defeat, blows himself up in a suicide attack, intending to kill the Mario Bros. but instead just blowing them out of Bowser’s body (source)