I'll write the story when I submit it to Mauve Manticore. You are supposed to kill the enemies. Hint 1: Spoiler Don't kill the burning fiend until you have killed the trolls. When you get retaliatory mutation, move close to the burning fiend. He will shoot you, which will give you fire attacks you can use against the trolls. Hint 2: Spoiler When you are getting fire attacks, only get two or you will lose retaliatory mutation. If you keep retaliatory mutation it can get you even more fire attacks, when the fire attacks that are attached to you burn you.
Its a great mechanical synergy, but in the end there's really not much challenge to it. Maybe alter things so that its harder to keep the two of them together, or that your more surrounded or otherwise pressed for time? I beat it at a mechanical, leisurely pace, and was never really threatened by the Trolls at all - my HP got low only when I decided to let it, and the priest never even was in attack range of an opponent. I love the design, but I'd love it even more if you'd ratchet up the difficulty a little.
Made a new version. Every square is enemy victory square. (Which means I had to get rid of the difficult terrain.) There are no victory points for killing. Which means you only have 10 rounds to win.