The Master Crown is known to change its overall shape throughout the course of the game. It comes in four forms:
When atop Landia's head, it resembled a typical crown with spikes extending from its jewel and base.
When Magolor first tried it on, it morphed into a larger crown, wherein the base spikes resembled dragon claws and the jewel spikes weren't present. When Magolor transforms into Magolor EX the jewel on the crown turns red instead of its original greenish blue color but is otherwise unchanged.
When Magolor transformed again, the crown grew and wrapped around parts of his body; the claws remained intact, but the top of the crown changed, no longer resembling a typical crown, featuring four protruding structures and some of it grew out and formed Magolor's wings.
When "worn" by Magolor Soul (seeing how it has now possessed Magolor it is debatable whether or not he can be said to be consciously wearing it), the crown changed into a white crown with two additional spikes extending from the side, two curled spikes extending from the back, and black spikes protruding from many parts of the crown. The jewel also changed color, bearing red, purple, and orange shades.
The Master Crown was first seen when Kirby, Magolor, and co. entered Halcandra via the Lor Starcutter, resting atop Landia's head and remaining dormant until the battle against Landia. After Landia's defeat, the Master Crown falls from its spot onto the ground, only to be seized by Magolor. Magolor then reveals that he fought Landia himself for the crown, but he lost, later fleeing to planet Planet Popstar— this is likely why Landia attacked the Lor Starcutter on sight when he returned to Halcandra. The crown transforms Magolor into a ghastly, jester-like creature as he makes his move to conquer the entire universe, starting with Popstar.
When Magolor is defeated, the Master Crown manifests itself as him. In Main Mode and The Arena, this entity is Magolor's third form; in Extra Mode and The True Arena, however, Magolor Soul is created. After defeating this villain, the Master Crown shatters, releasing its grip on Magolor. He then fades away, going to a dimension beyond space and time.
In the Team Kirby Clashsub-game, Landia serves as the opponent in Quests 5 and EX. It wears the Master Crown in both battles. This is because its 3D model was reused from Kirby's Return to Dream Land.
The Master Crown is also present in the standalone Team Kirby Clash Deluxe, resting atop one of Landia's, Landia EX's, and Parallel Landia's heads. Parallel Landia's version of the crown is black with a red gemstone.
The Dark Lord Void Termina uses two swords almost identical to the Ultra Sword used by both Kirby and Magolor, with a hilt very similar to the Master Crown in its first phase. In its third phase, it also creates/invokes a light blue image of the Master Crown to fire lasers at the Star Allies Sparkler, or, in The Ultimate Choice, a near perfect replica.
Magolor deceived Kirby and stole Halcandra's legendary treasure, the Master Crown! It's up to Kirby to save the universe.”
— VS Magolor • Kirby's Return to Dream Land
Magolor stole the Master Crown and the Lor from their sacred resting places on Halncandra. He's a liar and a wizard... Defeat him!”
— VS Magolor EX • Kirby's Return to Dream Land
A sad shell possessed by the limitless power of the Master Crown, no more than a manifestation of the crown itself.”
— VS Magolor Soul • Kirby's Return to Dream Land
Blinded by the power of the crown, the dragon attacked. Bring him back to the light!”
The Master Crown is likely a sentient entity. Magolor Soul's VS quote says it is "no more than a manifestation of the crown itself". This idea is supported by the fact that Magolor didn't actually put on the crown, but, rather, it floated to his head after Magolor held it in his hands. While sitting on Magolor's head before the battle against Magolor Soul (and by extension its normal mode counterpart), it can be seen occasionally wiggling and moving about on its own, perhaps trying to possess (or communicate with) Magolor.
Interestingly, in all of its forms except for its first, an eye-like pattern is clearly discernable around and including the crown's jewel, perhaps as a subtle indicator of the crown's sentience, an implication of it being omniscent (or all-knowing), or even implying that it is guiding Magolor's course of action. If the latter is correct, this also implies that Landia was not possessed by the crown at any point, suggesting that part of the reason Landia was guarding it was to prevent the crown from attempting universal domination (as it seems to have been trying to do when it possessed Magolor Soul).
Assuming it is indeed sentient, the Master Crown is one of the few items with sentience in the series to not help Kirby, arguably posing as a main antagonist instead by possessing Magolor and forcing him to do its bidding.
It is also likely that the Master Crown itself is not sentient, but something else controls those whom put it on. The most likely candidate for this is Void Termina, as it creates replicas of the Master Crown during its battle, and the eye in Magolor Soul's mouth is similar to Void Termina's own, as is the eye pattern around the jewel of the Master Crown.
The Master Crown manifests itself as Magolor's third form.
The Master Crown manifests itself as Magolor Soul.