Shadeys were an extra-dimensional species who used negative energy to give themselves power, and could possess individuals to manipulate their inner darkness. According to the Ninth Doctor, the Shadeys were "something evil", and told Rose Tyler to "take a look at them", after mentioning that some people didn't believe in evil and thought there were "no moral absolutes".
The Shadeys always tried to get into the Doctor's dimension to rip his time and space apart, but the Time Lords put up defences to keep them out. When these defences began to fail, two Shadeys, Uncle Bloodfinger and his nephew Woodscrape, broke through, and scouted out poet and playwright Robert Greene as he was succumbing from a fatal plague. Bloodinger and Woodscrape showed Greene the future, where William Shakespeare had become remembered, while Greene was forgotten. Stirring his bitterness towards Shakespeare for upstaging his popularity, they possessed Greene and tempted him to murder the playwright. The Ninth Doctor and Rose convinced Greene to expel the Shadeys from his body and banish them back to their dimension with the promise they would remember him after his demise. (COMIC: A Groatsworth of Wit)