Concluding an essay is always one of the hardest parts of writing it; even when you have a solid thesis and you have supported all your claims with citations and sources, it is hard to write a single paragraph that sums up all your work.
In the case of compare/contrast essays, such as one detailing the differences and similarities between the historical Trojan War and the 2004 film Troy, all the comparison should be completed in the body of the essay. At the point when you reach the conclusion, you should already have all the information you need to sum up your thesis, whether you have proven it or disproven it. For example, if you believe that the non-supernatural events shown in the film are a better representation of the historical events, your conclusion should repeat some (not all) of the information in the body, enough to clearly show how your thesis is represented, and finish with a sentence that asks for more research, since the history is still under dispute.
Another important point is not to simply repeat everything from your introduction; the point of the conclusion is to satisfy the reader that you, the writer, have made all possible effort to cover the topic at hand. If you end with sudden, new information, the reader will wonder why you didn't use that in the essay itself. If you go off on a tangent, there will be no connection with the essay and the reader will wonder why. Always try to keep the conclusion reasonably short, so the body of the essay remains the most memorable part.