Let's say I target a hero with a fire breathing monster. I roll a dice and get a surge, which is then used for fire breath. The second hero then rolls defense dice.

At this point, is it too late to use Dark Might to add a surge to the attack for the purpose of increasing damage against the hero effected by fire breath? Or do I have to use the Dark Might prior to using the surge that I rolled for fire breath?

It is too late. Dark might should be played after rolling dice- before ANY surges are spent.

The order should be:

-roll dice

-do any "after rolling dice" abilities (including dark might)

-begin step 3. check range, spend +range surges only if necessary to make it a hit

-begin step 4.

-spend a surge

-spend another surge (lets say this one is fire breath)

-trace path

-other affected heroes roll defense

-spend a surge

-move on to step 5, deal damage to heroes in the order you choose.

Any time during step 4 you can spend surges or trigger affects that occur while spending surges, but not after rollong dice abilities (the extra heroes rolling does not count for the attacker as a new trigger, but the defenders could use dice abilities here because this is their only chance.)

Thanks, I forewent using the Dark Might because I wasn't sure, but it will be helpful to know this henceforth.

Edit: You mention in step 5 that I can do damage to the heroes in the order of my choosing. Does this mean that I can start with the last hero hit by the fire breath? Could I then hit Leoric first to knock him down and nullify his aura for the rest of the targets? Or does his aura still count?

His aura would still count because the entire attack would get -1 to the attack results before any damage gets dealt to anyone. However, it has been confirmed that you could, for example, take out a reanimate first so the necromancer can't protect himself with dark pact.

On an unrelated sidenote, wiley OL thinking like that might have SOMETHING to do with the heroes getting creamed.