Dark Ring

Dark Rings (Evil Rings) are evil devices used by the Digimon Emperor to control digimon and make them his slaves. These devices are powered by his Dark Digivice, and its power is in turn spread by Control Spires. In order to free an enslaved digimon, the Dark Ring or a nearby Control Spire must be destroyed. They are not unlike Devimon's Black Gears and carry out a very similar purpose.

Dark Spirals
Dark Spirals (Evil Spirals) are similar in appearance to Dark Rings, but are spiral in shape and can control much more powerful Digimon than Dark Rings. However, they are more complicated and apparently more difficult to produce. Dark Spirals also don't need any Control Spires nearby to function.

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.