This article should be deleted since there's already one that's more detailed. Refer to Nabonidus for everything about him.
Nabonidus is one of the two nightmare in charge, alongside Lord Void. Previous to Lord Void's return, he was running the Nightmare force in secret for over a Millenium.
Nabondius seems to prefer control of the Dreamkeeper world over simply destroying it, which is the standard goal of most Nightmares.
Nabondius has shown the ability to teleport himself and others through shadows, see the nature of a Dreamkeeper's power before they are even first used, and seems to be immortal.
Nabondius is usually dubbed 'Nabby' between Dave and Liz Lillie when working.