|“||It was not benevolent. It was a frightful star that hid a great evil.||„|
|~ Princess Peach, about the Dark Star.|
The Dark Star is an archaic artifact of ultimate power that was sealed away beneath Toad Town and the overarching antagonist of the video game Mario & Luigi: Bowser's Inside Story, working in a parasitic relationship with Fawful.
Long ago, archaeologists unearth a mysterious artifact that housed terrible evil. The artifact soon came to life as the Dark Star and began attacking the Mushroom Kingdom in an attempt to plunge it into darkness. To protect the kingdom and the world from the Dark Star's power, Star Sprites sealed the artifact beneath Toad Town with Princess Peach being the only one capable of breaking the seal.
By the time of the game's events, Fawful finds out about the Dark Star and begins formulating a convoluted plan to obtain it, which he eventually does. Fawful then uses a machine to break the seal placed on the Dark Star and awaken its full power, which he planned to absorb using his Vacuum Helmet. However, when the seal was broken, Bowser knocks Fawful away after he managed to take half of it's power, allowing the Dark Star to enter Bowser's body and absorb some of his DNA. When Starlow sees the artifact, she immediately tells the Mario Bros. to go after it. After the pair defeat it in battle, the Dark Star exits Bowser and attempts to take his form, but is unable to fully achieve its new appearance without the power that was stolen by Fawful.
The Dark Star soon finds Fawful and inhales him, restoring its lost energy and finishes the transformation into Dark Bowser. It then kidnapped Princess Peach before fleeing to the top of her castle so it can once again try to plunge the world into darkness. Bowser unknowingly works together with the Mario Bros. to stop his dark doppelganger by forcing him to cough up his core (which had become attached to Fawful) and swallowing it for the bros. to destroy. Despite all his power, Dark Bowser was unable to prevent his core's destruction and was defeated by Bowser in a couple of final punches. Afterwards, Dark Bowser reverted back into its Dark Star form before it soared into the sky and exploded, saving the world from its evil.
At first the Dark Star could only express its personality through actions alone, but after becoming Dark Bowser, it became capable of voicing it’s desires. In any case, the Dark Star has a dark and almost emotionless personality with a great desire to plunge the world in darkness.