Decay of the Angel

The Decay of the Angel was a terrorist organization that specialized as a murder association. Although few in number, including Kamui, Bram Stoker, Fyodor Dostoevsky, Nikolai Gogol, and Sigma, its members all have powerful, threatening Abilities.

Description
The Decay of the Angel was a terrorist organization that has long since been hiding in Japan, operating as a murder association which possesses a single page from the Book. Mushitaro Oguri explains that the Decay has a fourfold scheme with the penultimate goal of eliminating the country, which they operate on by writing on both sides of the page.

Cannibalistic Mutual Destruction Operation
The plan begins with Fyodor's Rats in the House of the Dead first launching the Cannibalism virus that almost plunges Yokohama's Armed Detective Agency and Port Mafia into a blood battle with their respective leaders' lives on the line. The first stage ends with Fyodor's arrest and the rewarding of the Agency by the government with the that acknowledges their contributions to Yokohama.

Armed Detective Agency Frameup
A month after laying low momentarily, Gogol makes the Decay of the Angel's presence known to the Armed Detective Agency through brutal, horrific murders of four government officials within a single week. As horrific as they are, they are symbolic of the Buddhist Cycle of Existence. Having shown four of the five "", the Agency deduces that a fifth murder will soon take place. This sets up the Decay's second stage, which is to frame the Agency as a terrorist organization.

With the Decay's intelligence and abilities, Gogol also succeeds in infiltrating and impersonating government members of the Special Division for Unusual Powers, securing intel and cover-ups of deeper details of their actions later uncovered by Ango Sakaguchi. Even so, they've manipulated and swayed members, including the Ministry of Justice's vice minister Tonan, and likely several others.

After Gogol captures Mushitaro and the Decay holds him in their clutches, his ability The Perfect Crime stops working. Evidence of several hundreds of crimes come to light, especially those of the Port Mafia. This leads to the arrest of Osamu Dazai over his days as a Mafia executive. Moreover, with their hold over several government departments, the Decay of the Angel targets the Armed Detective Agency's downfall and total destruction.

Eventually, Gogol holds Tonan and various government officials hostage, an incident that goes awry for the Armed Detective Agency due to the power of the Book. The Agency is identified and successfully named as the terrorist Decay of the Angel. Concurrently, Sigma meets up with and attacks the Special Division director Santōka Taneda. Using Sigma's ability to exchange information the Decay wants to know most with information their enemies want to know most—that is, a trade of vital information—the Decay consequently acquires information on the Book.

This results to the Agency getting hunted for arrest by the military police and later, the government's elite hunting forces, the Hunting Dogs. At this point, Gogol is supposed to have died to achieve "total freedom" as originally planned, but he deviates from the plan and fakes his death in secret using his Ability, later enlisting Sigma's help to know Fyodor's Ability and killing him for his own personal plot.

Coin Bomb Terrorism
Later, Sigma is questioned in the Sky Casino by Hunting Dogs. This transpires into an airborne battle, which results in the Hunting Dogs witnessing Agency members and allies saving Sigma from falling and eventually, Sigma's surrender. Various explosions throughout the world are soon reported to be caused by the circulating coin bombs from the Sky Casino, which marks the third stage of the Decay's plan.

As the world takes notice of recent events, various foreign VIPs gather in Standard Island to forge an international corps to combat global terrorism, which is to be led by the legendary war hero and Hunting Dogs commander Fukuchi. However, Fukuchi's hidden persona as the Decay's boss is soon unraveled by Ranpo Edogawa, and Kamui soon admits that his motive is to take down all nations to bring an end to all wars, which is to be preceded by eliminating the country.

Vampire Infection Outbreak
In order to do so, Fukuchi awakens the Bram and uses his infection-type ability to turn others into vampires. The infection spreads from the Mafia all throughout foreign nations, including the Republic of Nalis, amounting to a total of 16 armies being ordered through the infection virus to disarm themselves.

As word reaches the United Nations about Nalis' withdrawal from their security council (along with various other nations), the United Nations secretary-general decides to contact the Order of the Clock Tower into sending in the ability weapon, One Order.

Trivia

 * The Decay of the Angel gets its name from 's novel . The reincarnation of Kiyoaki in the story is a manipulative and sadistic orphan named Tōru Yasunaga which may explain the group describing itself as a murder association.
 * The title Tennin Gosui specifically refers to the death stages of a within Buddhist canon which are represented by the murders they commit during their introduction.
 * The anime's adaption of the Untold Origins Arc heavily implies that they were also the behind the group V, as Fukuchi and Fyodor have been added in. Fyodor explains that V means "Five".