Extra: Black and a Mask

Extra: Black and a Mask「外伝: 黒と仮面, Gaiden: Kuro to Kamen」is the twenty-fourth episode of the anime adaptation of That Time I Got Reincarnated as a Slime.

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

Shizue Izawa 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 demon world, eventually finding Rimuru; where it is revealed that this Demon, 'Noir', shall be later summoned by him and be given the name 'Diablo'.

Summary
On a dark night, a woman, bloodied, battered and sat next to the corpse of her dead companion, is using her own blood to complete a ritual. The ritual in question summons a Demon. The Demon is asked to eliminate 'them' with the soul of the woman as payment. The Demon agrees with the task but states that her soul alone will not be enough, thus he will also feast on her prey.

Meanwhile, a hoard of Adventurers, including Shizue Izawa have been brought to the Nation of Filtwood, a kingdom that borders on the Jura Forest. Inside of the castle the high priest explains that they have a problem, a powerful demon is attacking their nation seeking to reclaim its corpse that is buried deep beneath their castle. Once it has completed it's revival it will go on to threaten the world.

Long ago this demon was defeated in battle by their nation's hero, Orthos, unfortunately, the demon has no name to go by and it's abilities or the exact location of its corpse have all been lost to the passage of time. They do know however that this demon has already defeated the Silver Wings, adventurers of considerable strength. With no information and no way of knowing how to defeat this demon, the adventurers are naturally a little concerned.

When one adventurer attempts to leave he is cut down and a lesser demon emerges from his corpse that the knight's quickly dispatch. Their enemy is already scouting his competition by using demons to possess the adventurer's as double agents. Thus before they can let them leave to find the Demon threatening their kingdom they need the Adventurer's to sort out who has and has not been possessed.

Towards this end, one of the Adventurer's begins laughing explaining that completing his request will be easier than he thought if all he has to do to eliminate all of the demons in that room is slaughter all of the people there. Having just said something she can't ignore Shizue fights the man who only refers to himself as Kuro, and who reveals himself to be another demon.

Shizue fights the demon to a standstill, but her Anti-Demon Mask manages to blow the Demon's arm off when he strikes it so he chooses to retreat. Overnight Shizue recalls on information told to her by Leon Cromwell about demons known as the Progenators, though they have no names for convenience sake they were given monikers based on the color of their aura's: Black, White, and Red.

Thinking about how Kuro means Black Shizue's ominous feeling about the demon she fought only grows. When morning arrives a Knight arrives in her room and explains that the king requests her presence in private. Meeting with both the King and the High Priest the Knight reveals that he is the true ruler of Filtwood and the demon that Shizue has been called there to defeat.

The story about the demon who attacked their nation was a lie, a legend staring the Knight as both the demon and the hero who defeated him, because of that collective belief the Demon has become self-named, Orthos. Having evolved into an Arch Demon he now seeks more power, thus he intends to feast on all of the adventurers gaining their power for his own and evolving into an even more powerful state.

They have isolated Shizue because they intend for her and Ifrit to become the Demon's first meal. Easily sub-doing Shizue who is still weak with the battle with Nero she almost unleashes Ifrit to have it defeat Orthos. However, Kuro suddenly arrives with the heads of numerous demonically possessed knights and easily kills the King, the Priest and Orthos devouring their souls to ensure that none of them are able to revive.

The next day Shizue spreads a rumor that the King and High Priest were killed by Kuro and she, in turn, killed him, meanwhile Kuro has finished purging the nation of all of the demons living there. Shizue knows she's not powerful enough to defeat Kuro, instead, all that she can do is hope that someone appears eventually who can defeat that demon.

Years later Kuro is shown have spotted Shizue re-enter the Jura forest, and viewing her as interesting watched her movements closely, including getting taken over by Ifrit and their subsequent battle with Rimuru Tempest. After Rimuru defeats and eats Shizue The Demon continues to watch the little Slime until the day finally comes where he summons him himself and is given the name Diablo.

Characters
MAIN: BRIEF APPEARANCES / CAMEO: NEW CHARACTERS:
 * Shizue Izawa
 * Diablo
 * Rimuru Tempest
 * Ranga
 * Orthos

Adaptation Notes

 * This is an anime original episode as it does not adapt any material from the Light Novels and the Manga.