Oh boy! Here's the ones from Splitting Image that I have art for so far.
Dark Lord Nycticorax is the "main" villain of the comic as far as the story goes. Their name, species and even gender is unknown, always clad in dark armor and a feathered white cape, with a shaky grasp of black magic but great abilities with their blood-red sabre. Their magic is similar to Mortimer's; but there's clearly a lot of learning happening to compensate for the sheer violence and purpose with which they attack our main group and temples around the kingdom - green around the gills but just as dangerous.
Phantasmata is a zombie-like villain summoned by Dark Lord Nycticorax during their attacks, and is far more experienced. She was once summoned by Mortimer as Dark Lord Mortallon for a distraction, and the glitter of mutal is an easy payment for her to wreck havoc on cities using her Nightmare lackeys.
She seems to attract them and put them under her thrall using a mysterious substance known as sable essence, a pitch-black material that changes from liquid to smoke easily and seems to put people to sleep in instants and reform into massive material forms of the victim's subsconscious, called Inner Shadows. With this, the Nightmares can feed on their dreams, and Phantasmata is free to steal and destroy whatever she needs, or collect data on the sable essence's effects for her endless research. She is no slouch fighting without her minions or sludge, however, and is a shadow mage who uses shadows to cheat and trap her way to victory.
The Gate Mirror... Is an intersting case, but it doesn't look much like a good creature, does it? A sentient old mirror that reflects what it faces and a little bit more, created to record black magic spells and enchanted to move on its own. It might've been created by a Dark Lord before Mortimer, and taught him to use things not found in books or ruined lairs and he used it as a focus more than once to seek out villains in the Realm Forgotten; and rather than leave its unknown location, it seems to torment the ex-villain for not spreading such occult knowledge to the world, just by existing. It might be serving something or someone that doesn't want to show their face...