Just a note to add. A Nazgul was thought to be in Dol Guldur in 1100 and it was 2060 before the power in Dol Guldur grows and is thought to be Sauron taking shape again.
Perhaps the Nazgul were trying to contact their master or find a body for him. Or perhaps they were gaining some sort of intelligence though this contact? I know that "the Necromancer" turns out to be Sauron but could it be a title that Sauron assumes when he assumes power there. Outsiders would not be aware of a transfer of power although they noted the increasing strength of that power.
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