Mage Slayer + Story Circle

[Arsenal]

I have a Story Circle in play (red), with 1 white mana available.

My opponent has an attacking Wholly Thoctar with a Mage Slayer equipped to it.

If I tap my 1 white mana for Story Circle, choosing the damage from the Thoctar, what happens to with the Mage Slayer? I realize Mage Slayer isn't doing the damage, but I'm not sure what happens in that situation. Do I have to pay 1 white again?

[Skeggi]

:w:: The next time a source of your choice of the chosen color would deal damage to you this turn, prevent that damage.

Whenever equipped creature attacks, it deals damage equal to its power to defending player.
When your opponent declares his Woolly Thoctar who is equipped with a Mage Slayer as an attacker, Mage Slayer triggers and will deal damage. You can pay :w: to prevent this. Then, when Woolly Thoctar is unblocked you have to pay :w: again to prevent this damage.

[Van Phanel]

When your opponent declares his Woolly Thoctar who is equipped with a Mage Slayer as an attacker, Mage Slayer triggers and will deal damage.

Just a little correction: The Mage Slayer does not deal the damage, the creature does. Which means that with a Story Circle set on white you could still prevent all the damage.

That aside Skeggi is right, you have to pay :w: once for each chunk of damage that you want to prevent.

[Arsenal]

Ah. I knew that the actual Mage Slayer didn't deal the damage, but rather, the creature deals the damage. So, the combat damage is one source of damage, and the Mage Slayer trigger is a second source of damage, resulting in me having to pay 2 white mana if I wanted to stay unscathed. Thank you.