Sinful Key

A Sinful Key (罪鍵) is an object mentioned by Shikama Dōji, obtainable via a few methods, of which there are seven in total. Multiple keys can be possessed by a single individual.

Appearance
Their appearance is similar to a dagger, more specifically it resembles as a cross-shaped dagger. It's size is small just as the size of a normal dagger. When first obtaining one it glows emitting a light around the dagger for a single moment.

Events of 2015
When Saitō was having a long conversation with Guren. In the midst of it, Saito 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, receiving no response, Mahiru-no-Yo materialized next to Saitō .

Saitō however despite not being able to see or hear her, he sensed a presence, seemingly to recognize her, asks if she is there. Mahiru-no-yo apparently conforms 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 says it's none of his business, instead he asks Mahiru-no-yo but she is ignorant as him. Just then Saitō throws the Sinful Key at Mahiru-no-yo neck, even though Saitō couldn't 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, confusing 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, and asking 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 handing it over to him 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. While Ferid and Crowley walk across 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 Saito 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, he retrieve it from Mirai then departs. Guren arrives to where Saito and Shikama Doji battle, Saito asking for the sinful key, he tosses it over to him. Saito then brings out his own two keys, now having three sinful keys. Considering three are enough to seal him away and he is yet to manifest his real body. After Saito's betrayal who is misled by Guren by calling on Ferid whom he actives Jigenso to portal next to Saito, he stabs Saito with his own sinful key. Directly after, Guren, Mahiru and Noya each acquire a sinful key then surrounding Shikama Doji which creates a sinister power entrapping Shinoa.

Shikama Doji 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.

Powers and Abilities
The Sinful Keys are extremely powerful despite their size and they can be used for both offence and defense, if stabbed by one such as in the case of Saito, who is a former 2nd progenitor it is enough to break him completely and immobilize him. The combined might of 3 Sinful Keys are also powerful enough to seal Shikama Doji, a being who has a power close to a god. 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). If a Sinful Key is used by a demon, their power is enhanced. Saito wanted to have all the seven Sinful Keys so badly that he has been searching for them over a thousand years. proving their strength. When Mahiru-no-Yo was pierced in the neck by a Sinful Key from Saito, she attained his abilities to use chains and the Sinful Key simply disappeared.

Sinful Key owners
 * Saito - obtained 4 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 who has Sinful Keys within them
 * The 6th Trumpet
 * Mirai Kimizuki
 * Mikaela Hyakuya (possibility)