The Powers are the Valar, the gods or demigods who sent Gandalf and the Istari to Middle-earth to combat the forces of Sauron.
Gandalf and Sauron
Sauron vs Olorin 1on1
Perhaps these links will assist in answering the other part of your question.
-Angmar
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...where the instrument of intelligence is added to brute power and evil will, mankind is powerless in its own defence.
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