Ringwraiths are bearers of Vengeance Rings and orginally come from Universe D-674, home of the Lost Ringbearers. They are controlled by the holder of the Dominator Ring and waged a successful war against the Lost Ringbearers through a campaign of organized, controlled terrorism attacks designed to undermine the reputation of the Ringbearers and then a full-scale, surprise assault that destroyed Nova Valdris in the first strike, as well as overcoming the Lost Ringmasters.
During the war, one of their primary duties was to capture Defender Rings. Ringwraiths would force Ringbearers into a situation where they could not or would not escape, and then (usually using superior numbers), maintain constant pressure to fully drain the ring of power before killing the holder.
Ringwraiths were introduced in the "Origins of the Ringbearers" story, "Let Vengeance Ring" by Richard Lobinske As a result of the events in that fic, the Ringwraiths (as a collective group) never existed. However, three Ringwraiths in possession of Vengeance Rings still exist, as they were either outside the time-stream or in 'The Bleed' at the time of the 'reset' enacted by The Agency. The identities of these two are unknown at this time. The third was identified as Ian Llywd, a rogue field agent from The Agency in the story "Temporal Conundrum.
Some Ringwraiths, in order to invoke fear in their enemies and victims, invoke what they refer to as the Ringwraith Oath:
Our enemies fall, from low and high
To salted earth, through burning sky
And all shall know our Ringwraith's cry
Let Vengeance Ring - and all hope die!