Wraiths, revenants, and souls.

[Nnystyxx]

I quite like this idea, and here are a few more:
Maybe make the Wraith have the ability to "Possess" someone, and instead of being able to move them, makes them speak in either Sumerian or other spooky ways. Might add to the spook factor more than actual power. (Your body seizes up, and your limbs go limp. Your mouth starts to move as a dark presence moves into your brain!)

I kind of like the idea of maybe something akin to the blobs Nuclei for the Shadowgod, maybe you have 3-4 hidden around the station (You sense mysterious energy signals emanating from the corridors of this forsaken ship. Maybe there is hope, after all.) , and the crew has to find it? They could be like, the bodies of tortured victims, contorted into horrific energy pylons, feeding power to the Shadowgod, and each one destroyed weakens the Shadwgod, debuffing some of it's powers. It could take about 30 seconds to destroy or desecrate one of these Flesh Pylons. (You start to move the tortured masses of writhing flesh off of the glowing pedestal. Ew.)

I like the idea of maybe some of the "Nightmares" coming to life, being controlled by dead people. It fits in with the theme, really. (The Shadow Wendigo starts to howl! The Shadow Wendigo leaps at you through the wall! The Shadow Wendigo begins to vomit a black, acidic slurry!)

Your name is growing increasingly appropriate! More spooky Sumerian is always fun.