Complex sentence: He seems to be an extreamly powerful being easily able to "suck the life" from Lt. Yar however he is reveled to have weeknesses brought out by him confronting the truth behind his evil nature.
Split into simpler sentences: He seems to be an extremely powerful being, easily able to "suck the life" from Lt. Yar. However, he is reveled to have weaknesses brought out by him confronting the truth behind his evil nature.