According to Sharn: City of Towers, the Sharn Deneith Enclave has been compromised by the Dreaming Dark for at least 3 generations (Sharn: City of Towers, page 114). That's for most of the Last War. For most of the time, they act like any other Deneith enclave. However, one of the House Proctors in Wroat is beginning to get suspicious.
How to get the PCs involved: A campaign like this takes time. It's best that the proctor deals with new inductees into the house to gather evidence of Compromise. The PCs may do odd jobs for the house first to prove themselves (such as defeat monsters, or defend someone's property). As they progress through the house, the Proctor may feel it is time for the PCs to do her little request. It's simple, they have to infiltrate the Sharn Enclave and bring back proof of a Compromise situation. When they have proof, they are to bring it back to the proctor immediately.
If the PCs are Successful: House Deneith will be able to strike a blow against the Dreaming Dark and may go up against the Enclave in Sharn. To prevent fracturing within the House, like what happened to house Phiarlan, the Wroat Deneith might deliver this information up to the Kalashtar Atavists and allow them to handle the problem. The PCs will be rewarded. Any Deneith PCs will be rewarded with a Stronghold construction contract from the King and allowed to build a Deneith Stronghold somewhere between Wroat and Sharn. The stronghold may become a village overtime.
(SNEAKERS! is a lite version). A couple of War Movies uses this kind of plot, and not to mention recent spy movies. All the best movies that used this idea are Rated R for Restricted.