Broly's Ring (ブロリーのリング Burorī no Ringu) is a device used by Paragus to mentally control Broly. Paragus wore the ring on his right hand, while Broly wore the associated "crown" on his forehead. It was destroyed by Broly when he transformed into his Legendary Super Saiyan form during his confrontation with Goku.
The device is used to restrain Broly from transforming into his Legendary Super Saiyan form, but it also causes noticeable physical side effects while he is a Super Saiyan, as it tints his hair blue and purple instead of the form's signature golden color, while also dulling his skin pigment. However, these side effects disappear soon after the crown is removed, as seen in the beginning of Dragon Ball Z: Broly - Second Coming. It presumably also works under long distances, as Broly remained under Paragus' control even when he was dispatched with Vegeta to track down the Legendary Super Saiyan at Planet Todokama, which apparently took about half a day for them to get to and come back. Paragus also implies that the crown, in addition to mentally controlling Broly, also saps his power level to the extent that it is lower than Paragus' own power level, with Future Trunks also furthering this implication by mentioning, after learning of the existence of the Legendary Super Saiyan via the Shamo slaves, that Broly was a "toothpick", and ruled him out as being the Saiyan in question.
Under normal circumstances, the control device worked perfectly, with Broly never being able to escape his father's subjugation. This changed once he reunited with Goku, the person form whom he has a deep hatred. The traumatic memory caused Broly's power to rise beyond the crown's control. Every time after this initial meeting, Broly regained more of his freedom, making it more difficult for Paragus to control and suppress him. On the third time, the waning suppression came to a critical level, in which the ring on Paragus' hand gave a warning beep and glowed red. The crown is ultimately broken when Broly transforms into his Legendary Super Saiyan form after his hatred for Goku reached its peak.
After an incident in which a teenaged Broly punches his father Paragus in the face and blinds him in the left eye on his path of destruction, he realizes that he has lost whatever influence he might have had over his son. Fearing for his own safety, Paragus employs a scientist named Krang to create a ring and crown to be used as a special control device to keep Broly under restraint.
Upon their completion, Paragus narrowly manages to survive adorning a sleeping Broly with the crown, and then quickly succeeds in using the ring to activate the crown's mental control. This event also caused Paragus to plan on using Broly's power to conquer the universe, as well as have the perfect opportunity to get revenge on Vegeta for his father nearly executing them. Paragus later has his son destroy a large portion of the South Area to set the pieces of his plan in place, and presumably had him transform while attacking the Shamoians in order to sell the illusion. However, Broly enters a rage when he sees Goku during the events of the movie Dragon Ball Z: Broly - The Legendary Super Saiyan. As Paragus has trouble controlling Broly, he takes the control device to Krang and asks him to fix it, but Krang claims that the device isn't broken and that Broly must be acting out on his own. Broly eventually breaks the crown when he transforms into the Legendary Super Saiyan and goes berserk, attacking Goku and the others on New Planet Vegeta.
Video Game Appearances
Broly's Crown is the Z-Fighter Exhibit #10 in Dragon Ball Z: Buu's Fury. It is obtained after defeating Broly in Nataday Village. The in-game description for this item is: "This crown worn by Broly was once used as a mind control device by Broly's father Paragus."
Broly's Ring is a Potara-item in Dragon Ball Z: Budokai Tenkaichi 3. If a character has this item equipped, they cannot transform in battle. It is obtained by having a wish granted by Porunga. A similar device was developed by Dr. Gero to contain Broly in an effort to get revenge on Goku in a what-if saga of Dragon Ball: Raging Blast.
Broly's Ring is an Ability type D-Item in Dragon Ball Z: Tenkaichi Tag Team. Like in Budokai Tenkaichi 3, if a character has this item equipped, they cannot transform during battle.