Riven

Q: I have a [Riven Broken Blade] with Svellsongur attached who currently has no Buff. I standard move her to attack a single enemy unit at a battlefield.

Is it possible for me to stun the enemy unit? Why?

A:

Yes. While this looks like a cost with instructions, “spend a buff”, it’s actually part of the effect of the ability, not the cost. So you don’t need a buff to put the ability “choose one” on the chain twice. This allows you to both get and spend the buff

Yes. Riven's ability would trigger twice. The 1st and 2nd choices would be made. Then they can be ordered where:

1. Riven is buffed first
2. During resolution, the Buff is spent to stun the enemy unit.

Many people have asked why the Buff is not a cost that is spent during finalization, even if it follows the CWI templating. It is because the CWI does not appear at the beginning of the effect (basically not at the first sentence) nor does it follow "you/they may" (383.3.b.). You may remember from the Unleashed FAQ with Insightful Investigator on why 2XP was not required to be paid on finalization.

In my opinion, I don't think this is intentional. I believe the rules has a lack of coverage for triggered abilities that involve modal choices, where the choice made is the actual effect itself.

Nonetheless, this is the status quo ruling.