Who was The Power that struggled mentally with Galadriel at Dol Guldur?

[lathspell]

While Sauron was there as the Necromancer, even he didn't know that Celeborn's wood was only like forty miles west of him.

There's the only point of critism, I have, Keeper of Dol Guldur. Sauron certainly knew that there was an Elven realm there, though he didn't know what was holding the extreme power as Galadriel says in LotR.
Gandalf says (in either 'the Shadow of the Past' or 'the Council of Elrond') that he was now almost sure that Sauron's original plan was attack Lothlorien and Rivendell when his host was large enough. The only thing that interrupted was the White Council, driving him out of Dol Guldur.

greetings,
lathspell