Decay of Angels (天人五衰, Tennin Gosui?) is a terrorist organization consisting of five members, including Ōchi Fukuchi, Fyodor Dostoyevsky, Nikolai Gogol, and Sigma. They 'specialize' in being a 'murder association'. Although few in number, its members all have powerful, threatening abilities.
Nikolai Gogol tells Mushitarō Oguri that the Decay of Angels is a terrorist organization that had long since hidden in Japan, operating as a 'murder association'. He murders the cab driver and captures Oguri. After this, the Decay of Angels lays low a month after the Rats of the House of the Dead's Cannibalism tactic. Making their presence known to the Armed Detective Agency through brutal, horrific murder of four government officials within a single week. However, as horrific as they are, they are symbolic of the Buddhist Cycle of Existence. Having shown four of the five 'angel', it is implied a fifth murder will soon take place.
With their intelligence and abilities, the Angels, particularly Gogol, succeeded in infiltrating and impersonating government members of the Special Ability Department, 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 one of the Ministry of Justice, and likely several others.
With Oguri in their clutches, The Perfect Crime stops working. Evidence of several hundreds of crimes come to light. Especially those of the Port Mafia, leading to the arrest of Osamu Dazai over his days as a mafia executive.
With their hold over several government departments, the Decay of Angels targets the Armed Detective Agency's downfall and total destruction.
The Special Abilities Department's Director Taneda was attacked by one of the members, Sigma. He has the ability to exchange information the Angels want to know most with information their enemies want to know most. That is, a 'trade' of vital information. Consequently, the Angels now have intel on the 'Book'. 
It was later revealed that the Decay of Angels also possess a page from The Book and have used it in order to frame the Armed Detective Agency and turn the entire government against them using the Page's power. It was later revealed that the Page is currently held by its leader, who is currently mobile.
- The Decay of Angels gets its name from Yukio Mishima's novel Decay of the Angel.
|Armed Detective Agency · Decay of Angels · Hunting Dogs · Mimic · Order of the Clock Tower · Port Mafia · Rats in the House of the Dead · Special Division for Unusual Powers · The Guild · The Seven Traitors · Transcendents|
|Sheep · Zoopark · The Seventh Agency|