She was probably there before Sauron moved into Mordor. I think he noticed that his Orcs started vanishing around her lair, and went to check it out, or sent someone to. I don't think they had an official pact or anything, it was most likely unofficial.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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