Here are few suggestions for what the “shadow creature” might be. None of these are necessarily the “true” answer, they’re just possibilities.
Maybe it’s a dark Force user from long ago, trapped by the Jedi and imprisoned in some sort of Phantom Zone-like netherworld for his crimes. He’s manipulating Vader in order to get free, and is using the Empire to have revenge on the Jedi Order and Republic of old that imprisoned him.
Or similar to the above, but he’s an alien from a long-vanished stellar empire who survived due to his imprisonment, and is using the Empire to lay the groundwork for his own conquests, while working to get himself freed from his prison.
Or maybe it’s an evil spirit, a malign Force ghost that is bound to an ancient artifact Vader discovered.
Or perhaps it’s a being from another galaxy or another dimension, one that views Vader’s rule as a stepping stone toward invading and conquering the GFFA. (This reminds me of how Nom Anor was portrayed in the run-up to the Yuuzhan Vong old-EU storyline: see for instance the Crimson Empire comics.)
I should note that in the 1975 third draft, the “Sith Knights” were not necessarily an ancient order. The idea seems to have been that when the Emperor seized power, many Jedi rebelled against him and were killed, but others remained loyal to the nascent Imperial government and became Sith Knights. Rather like the Inquisitors of current SW “canon”.
In Lucas’ notes he also considered having “only seven Sith - one in each sector”, which goes with the Inquisitors being addressed by numerical rank.
However, in the second draft, from earlier in 1975, there were multiple Emperors over time, due to a gradual corruption and decline of the Republic, rather than a single power-hungry tyrant. This idea lingered into Alan Dean Foster’s novelization, via a reference to “the later corrupt Emperors”. It’s probably also connected to an idea George Lucas mentioned to Foster and Charles Lippincott of having Leia become Empress at the end of the trilogy.
The second draft goes a bit more into the origins of the Sith Knights: a runaway “Padawan-Jedi” named Darklighter taught the Dark Side of the Force to a group of “Sith pirates”. “Sith” is an alternate spelling of “Sidhe”, so you might think of these as basically Space Elves. Apparently these Dark Side-wielding elf pirates were the nucleus of the Sith Knights who served the Empire when it arose later.
But again, some Sith were probably Jedi who turned rather than become victims of the Purge - this draft also mentions that some Jedi were tried by the corrupt Senate & executed. (From Lucas’ notes also is the mention that “Sith Knights look like Linda Blair in The Exorcist.”) The Sith Master, Prince Espaa Valorum, is Vader’s boss, and his residence is said to be on the Imperial cloud city of Alderaan, the floating prison from which Luke rescues his older brother Deak.
Plus, the second draft harps strongly on the notion that the Force runs in families - there were “several hundred Jedi families” before the Purge, while Luke’s father, the Starkiller (who is “over three hundred years old”), has trained at least seven of his sons to fight against the Empire. It wouldn’t surprise me if Lucas was setting up the second draft’s Valorum to be a secret Starkiller, another of Luke’s brothers.
The 1974 rough draft is more like the 1975 third draft in terms of treating the Sith Knights as simply fallen corrupted Jedi, who chose to serve the Empire rather than rebel against it. But in that draft the previous virtuous government was also an Empire, albeit a benign one - whereas the “New Empire” is evil, an aggressively expansionist state that has absorbed almost all the “independent Systems”.
…Also, in the 1974 rough draft there are no Force powers. At all. Jedi are fearsome warriors because of their intensive training, rather than telekinesis or mind-reading. There’s basically just a sense of premonition that is called “feeling the Force”.