I agree with Griton.
Either is allowed to wait and see what the other does and either is allowed to respond to the other's ability with his/her ability. This kind of situation creates so many awesome strategy choices that I can only conclude that its existence is intentional. For instance, the Runemaster can simply decide that he's not going to use Rune Mastery until the Uncontrolled Power is used up, but this runs the risk of not getting to use it at all. Conversely, the Overlord can choose not to play Uncontrolled Power until the Runemaster activates his extra surge, but then he runs of never being able to the play the card if the Runemaster is simply willing to live without his extra surge. It's a Princess-Bride-style stand-off, and it makes the game much more interesting.
Ultimately, the player who is more willing to let combat go through unmolested is in a better position.
It looks like you were right. I just got an official answer from Nathan at FFG.
What is the timing relationship between "when attacking" and "after dice are rolled"? For example, if a hero with the rune mastery skill attacks a monster, when can (or must) the overlord use a card like "uncontrolled power"? Must the hero choose to exhaust rune mastery before the OL plays the card, or is there a time after which the OL cannot play "uncontrolled power," but rune mastery can still be exhausted? Thank you.
“When attacking” is poor wording for timing. I would use/exhaust Rune Mastery after rolling dice. Because of this, both cards have the same timing. Either player can use their card before the other. If the Runemaster uses Runic Mastery after the overlord has played Uncontrolled Power, the overlord would still get to decide how to spend that surge.
Thanks for playing,
It's a standoff of wits and determination! I like it!