Defect

The defect, also known as the black shadow or bug, is a fault that was originally present in the Death God Verloren.

According to Ayanami, Verloren's reincarnation, the 'defect' is only found in reapers who stray from the duties bestowed upon them by the Chief of Heaven.

Anatomy
Like a Wars, the defect appears as a moving, black mass of pure darkness.

Wolfram Eifeler Barsburg
It seems that the Defect manipulated Wolfram into waging war on the kingdom of Raggs ten years ago. Wolfram seemed to greatly respect the Defect at first and told his advisors that thanks to the Defect, the Raggs War went well. However, in Kapitel 85, when Teito pointed the light of the Eye of Mikhail at him, Wolfram started to regain his conscience, and it is likely that he no longer respects the Defect.

The Defect was later revealed to be Landkarte (Katsuragi), or at least something manipulating Landkarte

Creation
"No. You people don't know how terrifying that is. It will surpass any kind of darkness existing above ground presently, and become a demon who will swallow everything."

- The Chief of Heaven

The death God Verloren was created by the Chief of Heaven to preside over souls. His purpose was to sort the souls entering Heaven; allowing the pure souls to enter Heaven, but punishing the 'bad' souls by devouring them. The Chief of Heaven had created Verloren to be emotionless, so that he could fulfil his reaper duties 'without having his heart disturbed', but despite the efforts of the Chief, Verloren and Eve (daughter of the Chief of Heaven) came to care for each other.

By experiencing emotion, Verloren 'strayed' from his duty and thus a 'bug' developed. This 'defect' would corrupt his body and turn his core "black like mud", but was also highly contagious, and, like a Wars, would infect everything and everyone Verloren came into contact with.

Containment
To prevent the 'defect' from spreading, the Chief of Heaven ordered Verloren to remain in isolation until he was cured, and informed Verloren that a remedy would be administered the next day. However, the defect had grown too severe, and the Chief feared for what Verloren would turn into once it took over his body. The Chief planned to have Verloren executed, but Eve intervened- allowing Verloren to escape to the human world.

The Chief of Heaven, then fashioned Seven heavenly lights out of Verloren's fragments and sent them after Verloren to deliver his punishment. However, the defect appears to have entered the body of Landkarte, resulting in the corruption of Katsuragi.

Defeat
During his fight with the defect, Teito Klein managed to purify Katsuragi, resulting in the defect's defeat and vanquishment.

Abilities and Attributes
Awareness: At some point, the defect appears to have become self-aware.

Harming immortals: The defect can harm, and sometimes even kill, otherwise nigh invulnerable creatures i.e. the gods and the Warsfeil.

It appears to have some control over its hosts or vessels, resulting in drastic changes in their personalities, to the extent that they lose sight of their original intentions and goals. It is highly skilled in emotional manipulation and deceit.