Dark Rings and Spirals
Dark Rings and Dark Spirals are the creations of Ken Ichijouji, as the Digimon Emperor. Dark Rings are black rings that tighten when they are thrown around Digimon. They allow the Digimon Emperor to control the Ringed Digimon (and the victim's eyes glow red too ^^). Dark Rings work on Digimon up to Champion level.
Ken invented the Dark Spirals when he couldn't control Digimon at Ultimate Level, like Andromon and SkullGreymon. They are exactly like Dark Rings, except that they are spiral-shaped and they can control Ultimate level Digimon.
More information: The Digimon Emperor