Extra: Black and the Mask

Gaiden: Kuro to Kamen (外伝： 黒と仮面)

A dying woman summons a Demon as revenge against the one who betrayed her and her comrades.

Shizue and many other adventurers come in Filtwood to exterminate a very powerful Demon, rumored to be an Arch Demon. Those not willing to fight the overwhelming foe are killed by the kingdom, due to which the 'summoned Demon' causes a ruckus and fights against Shizue. Realizing the mask she wore is an artifact that transcends space and time, the Demon 'retreats'.

At night, Shizue is brought to the throne room, where it is revealed that the Kingdom is under the rule of the Demon they were ordered to exterminate and that the whole commotion in the day was an act to lure and kill off powerful obstacles of the Demon, such as Shizue. The summoned Demon, 'Noir', reappears and kills the Demon as per his original contract, and makes a deal with Shizue stating that he had killed the Demon after which she had killed off 'Noir'.

Later on, it is found that 'Noir' had been tracking the mask from the Spirit World, eventually finding Rimuru. It's revealed that this Demon, 'Noir', shall be later summoned by him and be given the Name 'Diablo'.