The Dark Seal is a unique promotional item in Three Houses as it allows certification for characters to become a Dark Mage or Dark Bishop. Both classes are restricted to males. This is a rare seal and can only be acquired by either defeating or stealing one from the Death Knight.
Hubert and Lysithea have a diametric problem regarding Dark Seals: both can be considered the ideal units for the Dark Mage and Dark Bishop classes due to them learning Miasma and thus getting double the amount of uses due to the effects of both classes' Class Ability. However, Hubert is only playable in the Crimson Flower path, which features the least amount of chances to obtain Dark Seals, whereas Lysithea is barred from the aforementioned male-exclusive classes, and her native route, Verdant Wind, features the most chances to obtain Dark Seals.