When a hero use the surge ability of the Iron longsword and if the monster has 2 defense dice, who choose the dice to reroll? The hero or the Overlord?

Squeezy666 said:

Hero.

Good question.

I would tend to say that the user of the sword decides which die is rerolled - as the surge is used on his initiative.
But that is no more than my "guesswork" (and the way I instinctively played the card), as I believe that the question should be adressed to FFG.

More generally, the player who is using the ability that grants the re-roll is the one who chooses which dice to re-roll. If an ability lets you re-roll your opponent's dice, the intent is that it be used offensively - to make him re-roll good results in the hopes of making them worse.