The Dark Star is an extremely powerful and evil star seen in Mario and Luigi: Bowser's Inside Story. Dug up long ago, its primary purpose seems to be conquering the world.
The Dark Star was dug up and was too evil for the Toads in Mushroom Kingdom. The Star Sprites sealed it away deep under Toad Town to stop it's evil ways. It rested there until Fawful found it. Fawful planned to use its power to conquer Mushroom Kingdom. He used its power to barricade Princess Peach's Castle while he used the princess's power to break the seal on it. Just as the Dark Star's seal was about to be broken, Bowser appeared and fought Blizzard Midbus. Then Fawful used his headgear to drain the Dark Star's power, but only got half due to Bowser's interference and disappeared. The Dark Star then came to life and entered Bowser's body and began to absorb his DNA, apparently interested in Bowser's strength and other abilities. Though Mario, Luigi, and Starlow tried to capture it, it simply escaped by using one of Bowser's abilities. After three unsuccessful capture attempts Mario and Luigi engaged the evil star in battle. Though victorious, it obtained the DNA it needed and later on transforms into Dark Bowser.
The Dark Star has many powers to aid it in battle. It can clone itself and create dark satellites, spider-like creatures to assist it in battle and heal it. It can release a bright flash of light that causes the Mario Bros. shadows to come to life it then fires bolts of electricity at the shadow Bros. which in turn harms the real Bros. It can fire lasers at the Bros. It can also fling red and green energy blasts at them. Its strongest attack is firing a large laser at the Bros. When defeated, it will perform a desperation move where it sucks the Bros. in close and self destructs.