Other names include, but are not limited to, Alternate Moonglow, Time-Altered Moonglow, and Evil Moonglow.
Dark Moonglow is, as the name implies, a variant on the City of Moonglow, but burnt out, wrecked, and limited in size. It appears to be located in a kind of space-time bubble in the Ethereal Void. Various creatures are located in it that serve the Shadowlords.
The "time-altered" name comes from the possibility that this is a Moonglow from the future or from the future of some alternate reality. It is not clear at the time of writing (at least to this author it's not clear) if this is speculation or is actually based on something official.
Dark Moonglow is accessed through a black Moongate located near the Inn/Stable in Moonglow. This gate was created when the Blackrock Detector blew up, and is often referred to as a "Rift." It is similar physically to the gate that opens up in the Star Room when the Harrower spawns, and to the strange, non-functional Moongate currently present in Magincia following the Magincia Invasion event.