Ooh, free at last! I'm chock-full of power! I think I'll turn all this greedy sap's jewels into evil spirits! Bwa ha ha ha ha!!!
~ Black Jewel upon being reawakened.

The Evil Black Jewel is a treasure that Wario collected prior to the events of the game Wario World, and it also serves as the main antagonist and final boss of the game.

According to the game's intro movie, the jewel was once a huge threat to civilization, sending powerful armies of spirits across the land in an attempt to conquer the world. The Spritelings were able to overpower and capture it somehow, and seal it away in their ruins. One day, many years later, Wario found it and stole it, unbeknown to the dreadful powers it held.

On the night that Wario is celebrating his huge gains in his newly-built castle, the jewel re-awakens, destroys the castle, turns most of Wario's treasures into monsters and scatters the rest. Wario falls and lands in what has become of his castle; a strange hub-world with eight levels attached to it. Wario must beat these levels and their bosses in order to acquire the massive treasure key that will open the Evil Black Jewel's treasure chest.

As a boss, the Evil Black Jewel has three attacks. The first one is using its eye to fire a laser, creating a wall of fire. The Evil Black Jewel will use this attack until Wario is trapped between 2 walls of fire narrow enough. If the space is very narrow, the Evil Black Jewel will fire shockwaves across the floor which will have to be jumped over, similar to those that Bowser makes in the final battle of Super Mario 64 and Super Mario Galaxy except that these shockwaves are multi-colored. If the fire-walls are not very narrow, the Evil Black Jewel will fire fireballs, which are easy to dodge if Wario is very close to the Evil Black Jewel.

To defeat the Evil Black Jewel, the player will have to free the 5 Spritelings on the battlefield, which will smack themselves into the monster, knocking it unconscious. Wario must then pick it up and use any one of his attacks on it (the jewel is extremely weak; even a soft throw will deplete its HP). The Evil Black Jewel has 8 HP. Once this HP has been depleted, the jewel will let out a piercing scream before exploding into millions of pieces.