I think it should be: Мощность, который победил смерть, живет во мне.

который would be used in this case since it's a subordinate clause. You could basically remove everything in between the comma's and it would be able to stand alone as its own sentence.

который could be literally translated as "which". While "that" and "which" might be interchangeable in English, I think they are mutually exclusive in Russian.