If you read the shadowpriest FAQ at the top of the page, you will actually find a suggested talent progression to follow.
A lot of the questions you have, and will have as you progress, are answered in that FAQ. Some good people spent a lot of time to create it.
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"the people can always be brought to the bidding of the leaders. That is easy. All you have to do is tell them they are being attacked and denounce the pacifists for lack of patriotism and exposing the country to danger. It works the same way in any country."
Hermann Goering, April 1946.