If you have Valyndra's Bane (which gives an extra yellow power dice if you attack large monsters) and you attack with Whirlwind with both large and small monsters adjacent to you.

how many dice, you have to use in the attack??

Good question. You might need to ask FFG for clarification. I guess you could roll the red yellow and blue. Spend surges then roll an addition yellow die for the large monster. But what if your only surge is with the last monster. Then you only pierce the large monsters?

in my opinion, with whirlwing, you're making an attack, not declaring an attack, so you don't have the bonus of the yellow dice, since no monster is targeted personnaly

Whirlwind:

Action: Preform an attack with a Melee weapon targeting all adjacent monsters. You make 1 attack roll and each monster rolls it's defense dice separately. Cost: 1 Fatigue.

Valyndra's Bane:

Each time you attack a large monster, you may add 1 yellow power die to your attack roll.

Surge: Pierce 3

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You are preforming an attack against all adjacent monsters, if one of those monsters is a large monster you add one yellow power die to the roll. Valyndra's Bane only says if you attack a large monster (Which you are, it is one of the targeted monsters.) it adds the die, not that it must only target/affect the large monster. Whirlwind allows you to attack all adjacent monsters with a melee weapon.

in my opinion, with whirlwing, you're making an attack, not declaring an attack, so you don't have the bonus of the yellow dice, since no monster is targeted personnaly

The first step in making an attack is to declare the attack. There is no distinction there.

As SoulX said, I would only add the additional yellow die result to the defending large monsters.

the trouble comes from the word "target"

As SoulX said, I would only add the additional yellow die result to the defending large monsters.

Actually, it would add the yellow die to the whole attack and affect even the small monsters.

the trouble comes from the word "target"

There is actually no problem from this word here, Whirlwind allows you to attack all adjacent monsters. The addition of the yellow die happens when you attack a large monster. It's just one of the targets, giving you an additional yellow die to the attack.

the trouble comes from the word "target"

You said the difference was between making an attack and declaring an attack. You implied that "making an attack" does not involve a target, whereas "declaring an attack" does.

I responded by saying that making an attack and declaring an attack are the same thing. Therefore both equally involve the use of a "target." Per the attack rules on page 12, every attack must target a space. The target space must contain an enemy figure. This figure is the target figure. Therefore every attack "made" or "declared" (same thing) have a target.

Where's the trouble, specifically?

I think you're getting confused with attacks that "target" or "affect" a figure. Every attack in the game has at least one target, that being the figure in the target space. Some attacks are able to additionally "affect" other figures, which are not "targets." However, every attack has at least one target.

Valyndra's Bane does not use the word target, therefore the ability applies to any attack made with this weapon that affects a large monster. Whether that monster is the target or not, a monster which has been affected by an attack has been attacked, thereby meeting the condition on the weapon.

Whirlwind allows you to make an attack with a melee weapon that targets all adjacent monsters. Valyndra's Bane is a melee weapon, therefore it can be used with Whirlwind. If one of the adjacent monsters is a large monster, then you have attacked a large monster with Valyndra's Bane, and its bonus kicks in. The fact that said large monster is indeed a target is actually just cake - Valyndra's Bane would kick in even if the attack merely affected this monster.

Once the yellow die is added to the attack, it is added to the whole attack, which means the result of that die is applied to all monsters subject to Whirlwind large or not. Like a condition or a surge bonus being added to the attack, it applies to all figures affected equally.

Do note, however, that the bonus from Valyndra's Bane is a true/false check. It doesn't stack if multiple large monsters are being hit by Whirlwind. The condition is met for the whole attack whether you're hitting one large monster or 8 large monsters, and when the condition is met, one yellow die is added.

Interesting interpretation, Steve-O !

I am still not that sure that the bonus die would apply to non large monsters, but you have very good arguments in favour of that application of the rule.

I would venture that asking the question to FFG could be a nice idea.

Just got mail from FFG.

Hey Peter,

Thank you for the answer !

I hope someone will post it in the "FFG sez" and wiki on BGG.

Thank you for the answer !

I hope someone will post it in the "FFG sez" and wiki on BGG.

Well, I decided to be that "someone"...

http://www.boardgamegeek.com/article/12943760#12943760