Short answers, pulled from the story.
Astronomers at the W. M. Keck Observatory in Hawaii spotted Dysnomia on the 10th of September 2005. Mike Brown and his Laser Guide Star Adaptive Optics team identified this faint point of light orbiting Eris during their study of four bright Kuiper belt objects.
Mike Brown chose the name Dysnomia from Ancient Greek words meaning anarchy or lawlessness to honor the daughter of the Greek goddess Eris. This choice also echoed Lucy Lawless, the actress who played Xena on television before the pair received formal names as Xena and Gabrielle.
Submillimeter observations taken by the Atacama Large Millimeter Array in 2018 refined Dysnomia's diameter to approximately 870 kilometers. This measurement makes it span between 24% and 29% of Eris's total diameter while maintaining a very dark surface that reflects only 5% of incoming visible light.
Dysnomia completes one orbit around Eris every 15.786 days which equals roughly half a month. The average distance between the two bodies spans approximately 37,000 kilometers with an orbital inclination relative to Eris's heliocentric path measuring approximately 78 degrees.
Scientists believe frequent collisions between large Kuiper belt objects created these moons when impacts involving bodies thousands of kilometers across ejected massive amounts of debris. That material eventually coalesced into satellites orbiting their parent planets similar to the mechanism likely forming Earth's Moon when a giant impactor struck early in Solar System history.