The Majin Seal is the evil version of the Halo in the Budokai Tenkaichi series. Whereas only good characters can wear the Halo, only evil characters can wear the Majin Seal. This item allows the character using it to use Blast 2 and Ultimate Blasts by using only half the ki normally required.
Though the Halo sometimes appears on characters using it (such as Goku and Vegeta in Dragon Ball Z: Budokai Tenkaichi 2), the Majin Seal does not appear on any characters. However, some characters, such as Majin Vegeta and Majin Buu always have a Majin Seal visible, even when one is not equipped.