Target will be chosen randomly if it's a Hero Power like Fireblast.
And of course the opponent will pay the mana required to use his Hero Power, that's the point of the card.
Edit: The mana cost is back to 0 now but changed the condition for activation, now it will activate after your opponent plays a card and not at the start of their turn to give a chance for your opponent to play aroud the secret.
The secret won't activate if your opponent used their Hero Power before playing any card (unless they have a card like Coldarra Drake on the field) or if they don't have enough mana to use their Hero Power which make the combo with Saboteur less consistant.
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Skar'this the Heretic is a friendly naga who serves Neptulon.
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