Board Thread:General Discussion/@comment-4962906-20151111090025/@comment-3274978-20151112125107

Gaining the powers of the grail is a blessing from the gods, but that blessing turns into a punishment, a curse (the Dark One's curse), if the user it's tempted by the Darkness/Evil. All the Dark Ones' hearts and souls are consumed by this curse, infecting it with Darkness. Eventhough the Darkness existed before the curse was made, because all people (only the ones with a normal red heart) have the free will to be whatever they want: good or bad.

Nimue chose revenge over mercy. Thus, her blessing turned into a curse. A part of the curse it's having a skin as ugly as their hearts, a physical punishment to distiguish them from regular people.