In Victorian England it is commonplace for the rich and wealthy to have a staff, led by a head butler, to run their households; the Phantomhive Estate is no different. The young and demanding Count Ciel Phantomhive, child owner of a toy company, lives in the grand countryside manor. Sebastian is his head butler, and the epitome of perfection; he effortlessly and gracefully completes his day-to day chores and fixes the countless mistakes of the other employees. However, whilst on the outside all seems prim and proper, a more sinister secret lies just beneath the surface. Sebastian is in fact a demon bound by a contract with the young count; he will loyally serve and fight for him in return for his soul.
The connection here is pretty obvious; both manga deal with similar themes such as the underworld and feature a contract for survival between a younger man and a supernatural being. The victorian-esque art style is very similar, despite Sougiya Riddle taking place in modern times.
If you enjoyed reading one, then giving the other a try is a must.