An ancient artifact in the Kirby games, introduced in Kirby's Return to Dream Land. It is said to give its wearer unlimited power.
The main story of Kirby's Return to Dream Land is driven by Magolor's efforts to steal the Crown from its current guardian, Landia. After enlisting the help of Kirby and his friends, he succeeds in doing so and attempts to conquer the universe, starting with Pop Star. The Crown's influence and evil sentience get the better of him and he transforms in a monster that is swiftly defeated by Kirby and company.
Void Termina is able to summon the Crown (or at least a replica of it) during his boss battle in Kirby: Star Allies. This along with the Jambastion sigil closely resembling the Crown suggests a strong connection with him, Dark Matter, and the twisted treasure.
Kirby's Return to Dream Land Deluxe confirms that the Master Crown is in fact, sentient, having brought the downfall of multiple civilizations in the past. In the Magolor Epilogue, it first enhances the Grand Doomer into the Crowned Doomer, then assimilates the Gem Apple to turn into a tree monster before being poetically struck down by Magolor’s Ultra Sword.