Although it's presumed to be part of Dominiaria, Shimia is shrouded in mystery to this day. Shimian Night Stalker bears real-world flavor text and little in the way of background illustration, so the card itself doesn't provide much insight as to what Shimia is like. Certain old, internal Wizards documents make vague reference to the ability for Night Stalkers to raise undead minions. That ability is not reflected in its card mechanic but may suggest that more lore about Shimia was once under consideration.
Shimian Specter represents the rebirth of references to this little-known location, and certainly shares the Night Stalker's sense of odd creepiness. As you can see from the art, the specter is shown from above as it swoops up the side of a building. It holds a needle-like polearm that glows white-hot at the end, a reference to its mind-cauterizing attack.