I really like the idea of your backstory. Robot at war finds a creature that will grant him a wish. And I just can't help but wonder what would a robot ask for? Part of what I like about this circumstance is how you designed the robot. It doesn't look too intimidating or violent. It looks strong, but there's nothing here to indicate that it would want to hurt somebody. It's sort of... neutral. Like maybe if given the chance, it wouldn't want to go to war. But then again, we don't know. And that is a part of what matches up great with the scenario of the poster. Something neutral, vague, unknown, maybe innocent, given such a large power.
I like the detail of the robot, and the way the robot and the background are sort of gray and bleak pair up with the war theme nicely. And the contrast between the bleak robot and the pretty shine of the wish-granting creature is great. Almost like hope among despair or something. Great work.