Reposting this idea: Imagine Davy Jones being a Quarren Sith Lord piloting a feared ship across the galaxy, and the Kraken being a Summa-verminoth under his control, taking down star destroyers and other spaceships across the galaxy. Instead of souls, this Sith lord could instead drain the Force of the ones inside the ships to keep himself alive.
Quarren Rebel, like early Mon Cala rebellion before the alliance was a thing. Gets sent on a mission to try and attract enemy attention away for a different raid. Rescue never comes and he feels betrayed/abandoned but help never comes because the other mission failed and they are all dead, he escapes but never finds out what happened. Discovers his force sensitivity and falls to the dark side in his anger, becomes space pirate Sith Lord after discovering Holocron that teaches him how to control an ancient beast in an asteroid field, Nebula, lost system etc.
A more expansive version:
The legend of the Flying Dutchman was created, due to the dutch Merchant Fleet possessing the fastest ships at the time, resulting in dutch trading vessels being encountered all over the seas, so the supersticious seamen called it devilscraft.
Knowing the background of the Flying Dutchman, I'd say, go a similar way and call it after a Race that is known for trading. Therefore, since I couldn't find much else, for the time being, I'll call it the Neimodian.
As for it's looks, has anyone seen the animated movie Space Pirate Captain Harlock, from, I think, 2013? If I recall correctly, his ship, the Arcadia, was able to repair itself and often times was shrouded in a black, smoke-like cloud. Something like that. And knowing what is possible in the Star Wars universe, this could be plausible.
So, Darth Altus originally belonged to the old Sith Empire, maybe era of the Old Republic.
At some point, he disappeared from the stage, believed to have died. But in reality, he travelled to the outer regions, and now he has returned, more powerful than ever before!
There, deep in the unknown space of the outer regions, he meditated. He listened. And he learned. He aquired some skill in the powers used by Darth Nihilus, taking his own spin at it, and mastered it. Using forbidden Sith Alchemy and his Powers of the Force, he became something different. The ship he was on changed. Caused by the dark rituals he performed, the ship transformed, together with him. His ship, the Neimodian, became almost impossible to destroy, repairing every damage it took by itself, without the need of fuel of any known kind, and binding the Sith Lord to itself for eternity.
Darth Altus, in his Quest for power and knowledge, became a Force Vampire, feeding on the Force of the living beings around him, and even mastered the ability to, if he wished so, rip the Force out of a living being, permanently destroying it, preventing it from becoming a Force Ghost, but also to sustain a Form of live over years, keeping some of the crew of the Neimodian alive to manouver the ship and to feed upon.
There he also learned to control a Summa-Verminoth, which now obeys his every command.
After years in his self-exile, he learned by coincidence of the Fall of the Sith Empire, and for several more years, that was his last contact to the known Galaxy. Now he has returned to haunt the known Galaxy.
Being not entirely material anymore, the Neimodian is almost impossible to detect by normal means. And when it appears out of dark space, cloaked in what appears to be a cloud of black smoke and lightning, it's almost to late to run. Darth Altus himself can't leave his ship for too long, without risking to become weak and possibly die, due to being bound to his ship. But he still has his crew of almost undead, being loyal to noone but him, attacking and capturing other ships, leaving no survivors behind. It's better to die in the attack, than to get captured by the crew of that black ship. Those that survive the attack, are brought on the Neimodian. There, they either end as meal for the Sith Lord, who rips their knowledge and force from their bodies, killing them, or become part of the crew, getting fed upon, over time turning them into his loyal thralls. And those that manage to run, he sends his pet after.