what were The Silent Watchers?

[Keeper of Dol Guldur]

Sauron the Necromancer was master of fell spirits (obviously before the Necromancy days he was gifted in placing them into places where they didn't belong like into the werewolves). During his days 'in cognito' the seemingly endless supply of fell spirits was tapped again and many haunted Angmar and Carn Dum, and migrated to the Barrow Hills. They found their way into the dead, and reanimated the corpses. But it would seemingly be more easy to 'haunt', to place a spirit into a former living thing than a statue. That's where Sauron's talent as a master artificer comes in. He was good, he made the ring, under his watch the other rings were made, by his tutelage even the elf rings, and Barad Dur, Sammath Nuar, the great highways and lava channels, so obviously that was one of his projects put to good use, Sauron certainly didn't go to Cirith Ungol and build it, it was probably the work of a few trolls getting the obsidian statue all the way up that mountainside. That statue could've once guarded Dol Goldur, or anything else. There were probably dozens of them littering each of his major fortresses. He was very good at making these sorts of things.