Dark Ring

The Dark Ring (イービルリング) is a black ring, of varying size and shape, that appears in Digimon Adventure 02. Its design is derived from 's Holy Ring, and by placing the Dark Ring on a Digimon, the can completely control the Digimon's actions. A Dark Ring can only operate when near a working Control Spire, and only works on Digimon at or below the Champion level. Dark Rings are inscribed with DigiCode which reads Ibiru Ringu (イビルリング).

The Dark Spiral (イービルスパイラル) is an upgraded version of the Dark Ring, formed by connecting several Dark Rings together into a spiral. It can control Digimon up to the Ultimate level, as well as operate away from Control Spires. Dark Spirals are inscribed with DigiCode which reads Ībiru Supairaru (イービルスパイラル).

Digimon Adventure 02
After failing to digivolve the kidnapped Agumon into MetalGreymon, the Digimon Emperor realized that Dark Rings could only successfully control Digimon up to the Champion level. He then created the Dark Spiral and was able to make Agumon Digivolve into a virus-type MetalGreymon and control him with ease. It is unknown if the Dark Spiral could control any Digimon over the Ultimate level as it was never tested for this purpose.

Much later on, it is revealed by Myotismon that the design for the Dark Rings and Dark Spirals were based off of Gatomon's tail ring. The details of this are unknown.