Sinful Key

A Sinful Key (罪鍵) is what Shikama Dōji calls a cross-shaped dagger like object. There are seven in total and mentioned by others, they’re obtainable via a few methods. Multiple keys can be possessed by a single individual.

Background
Connected with seraphs the sinful keys are kept within the bodies of human hosts.

Sinful Key owners

 * Saito - obtained four by unknown means.
 * Guren Ichinose - obtained one after killing Mirai Kimizuki.
 * Ferid Bathory - obtained one after the 6th Trumpet was neutralized.

Seraphs with Sinful Keys
Currently very few Seraphs have Sinful Keys within them:
 * The 6th Trumpet
 * Mirai Kimizuki

Appearance
Their appearance is similar to a dagger, namely a cross-shaped dagger. Its size is small just as the size of a normal dagger with the blade longer and thinner compared to the grip and guards which are of similar length. The guards have a ringed feature on the end, the same as the pommel. When first obtaining one it glows, emitting a light around the dagger for a single moment but otherwise is completely black in appearance.

Capabilities
The Sinful Keys are extremely powerful despite their size and they can be used for both offensive and defensive purposes. Just a single Sinful Key can severely injure and weaken Saitō, a former second progenitor, dismembering and preventing him from healing at any timely manner, and likewise Urd Geales, who is indicated to be stronger than the former, was wary of the Sinful Key. The combined might of three Sinful Keys are powerful enough to seal Shikama Doji, the first progenitor who has acquired powers to the point where he is viewed as close to a god, though he could have resisted it had he wanted to. Sinful Keys have other mysterious powers but it is yet shown. It is unknown why a Sinful Key shows up when few Seraphs are killed or neutralized. They can send a mysterious sinister power to trap someone (such as they entrapped Shinoa who was possessed by Shikama Doji) and Ferid is aware that the keys have a range. If a Sinful Key is used by a demon, their power is enhanced. Saitō wanted to have all the seven Sinful Keys so badly that he has been searching for them over a thousand years, demonstrating their valuableness. A demon, Mahiru-no-Yo was struck in the neck by a Sinful Key from Saitō and she gained his abilities to use chains, then the Sinful Key simply disappeared.

Events of 2015
When Saitō was having a long conversation with Guren, in the midst of it, Saitō smiled and took a Sinful Key out of his breast pocket. Guren was clueless about the dagger object, questioning Saitō about his intention of using the dagger and receiving no response, Mahiru-no-Yo materialized next to Saitō.

Saitō however despite not being able to see or hear Mahiru still sensed a presence, seemingly able to recognize her so asks if she is there. Mahiru-no-Yo apparently confirms so, though she adds even if he knew he doubts he will be able to see nor hear her. Guren informs Saitō of her location, which Saitō had a feeling so.

Guren asks about the dagger, but Saitō replies it is none of his business and instead asks Mahiru-no-Yo but she is ignorant as him. Just then Saitō throws the Sinful Key at Mahiru-no-Yo's neck, even though Saitō could not see her. She gives a short cry, putting her hand on her throat and then she collapses..

Screaming at Saitō, Guren demands to know what did he do. Though unconcerned, as he hid something, this confuses Guren when Saitō mentions he hid a sin. Before departing he informs Guren that he will find out soon enough, as they need to collect those sins for their wishes is to be granted.

Events of 2020
When the 6th Trumpet is neutralized and is stabbed by an injection into the angel's neck, the 6th trumpet floats into the air and a cross-shaped dagger appears.

At the Sunshine 60 Building in Ikebukuro, Tokyo, Saitō is seen holding two Sinful Keys while pondering to himself how many thousands of years it took to get this far, then he asks himself about Sika Madu's plan while holding the two Sinful Keys.

After the events of the 6th trumpet, Mika takes hold of the key that came out of the seraph, Ferid requests he hand it over to him and after some argument Mika gives Ferid the key. Guren says they will be taking the sinful key to Tokyo so that humanity can fight back against "millennia-old monsters".

Much later the sinful keys play a big role in the battle against the First. As Ferid and Crowley walk amongst unconscious Hyakuya Sect Soldiers, Ferid is playfully balancing a sinful key on his finger. Crowley asks about it and what it does, and Ferid is aware it is a magical item that can do lots of amazing things. What is more interesting to him is that Saitō badly wants to have them all, and has been looking for them for a over a thousand years.

Later Guren come across Shiho who is carrying his sister Mirai and after killing Mirai a sinful key comes out of her, which Guren retrieves then departs. Guren arrives to where Saitō and Shikama Doji are fighting, Saitō asks for the sinful key and Guren tosses it over to him. Saitō then brings out his own two keys with three in total and considers three are enough to seal the first away, as he is currently possessing Shinoa and has yet to manifest his real body.

After Saitō's betrayal, he is betrayed in turn by Guren calling on Ferid, whom activates Jigenso's portal next to Saitō and stabs Saitō with his own sinful key. Directly after, Guren, Mahiru and Noya each acquire a sinful key then surround Shikama Doji where a power entraps Shinoa. The First leaves Shinoa's body and allows himself to be sealed since the power that was emitting from the sinful key would break his vessel; Shinoa.

When the Hyakuya Sect are retreating from the battle with the Imperial Demon Army, and chain marks in the ground are noticed by Ferid leading to an alley, he takes out his sinful key. It produces a sound and effect when scraped along a building wall and Ferid raises it in front of Saito before bringing it down on him. He keeps ahold of it even with his arm bound by chains and when his arm is sliced off and caught by Crowley. It remains in Ferid's hand and travels through Jigenso's portal to be thrown towards Crowley. After it is caught the sinful key activates with a wide area effect inflicted on Urd Geales, Lest Karr and Ky Luc. The sinful key's powers are negated by a loud window shattering scream that only vampires can hear.

Having kept ahold of his, Ferid regards a sinful key after perceiving that the First has been captured, as Guren does the same in a separate location. After Yu has returned with Mika who is now a demon, Guren holds his three sinful keys at his side. Tossing his up into the air, Ferid considers how he thinks it is finally their turn.