The Dark Star is a sentient artifact of incredible dark power featured prominently in Mario & Luigi: Bowser's Inside Story. Long before the time of the game, it was discovered by Toad archaeologists. It viciously attacked the kingdom, and was sealed away by the Star Sprites. It was said that if any great evil being broke the seal on the Dark Star and allowed it to attack the kingdom once again, the evil entity would receive enormous power. The Dark Star is the true main antagonist of the game and the second half of the game focuses on Mario, Luigi, and Bowser's attempts to stop the evil artifact from attaining its full might.

                                                                                                            wikia