Yes I know, I've obviously played the DB questline. The other guy said Cicero was still alive though, which is not canon. Canon in TES usually means maybe, because you can kill almost everybody at some point. Still, the Penitus Oculatus wouldnt just stop after 1 attack. The DB can't survive the Penitus, especially now that there are only 2-3 members left, and they murdered the Emperor. They're screwed. The Penitus Oculatus are assassin/spies as well, like the KGB.
Cicero is alive if you destroy the DB. Taking that route you can kill all but two members (Cicero and Babette). Joining the DB you can kill all but two members (Nazir and Babette) Cicero might be alive using the last option. I seriously doubt that the Penitus Oculatus is capable of killing the remains of the Dark Broterhood.
First of all their presence in Skyrim was because the Emperor was planning a visit, with the Emperor dead their reason for being their vanishes second their commander officer is either dead or probably under arrest for his big screw up in protecting the Emperor. Then the Dawnstar sanctuary that hasn't been used in hundreds of years is the new place to go, which is simply unknown to that group, and nobody will sell out the DB now. Equally improtant if the civil war has not yet been completed or has been completed in the favor of Ulfric then that entire organization has absolutely no power in Dawnstar. The DB will live on, as always.