Card text of A Test of Will:
"Response: Cancel the "when revealed" effects of a card that was just revealed from the encounter deck."

Card text of Sacked!:
"When Revealed: Attach to a hero ...
Shadow: If attacking enemy is a Troll, resolves this card's "when revealed" effect."

During the attack of a Hill Troll (which has the trait Troll), the shadow card dealt is Sacked!, so its "when revealed" effect is to be resolved. Can I play "A Test of Will" as a response to it? Although A Test of Will just says "effects of a card" instead of "effects of an encounter card" (which a shadow card is not), is the shadow card considered "just revealed from the encounter deck"? At first, I don't think so, as in rulebook p.18, the shadow card is dealt at the beginning of the combat phase and is flipped faceup during step 3 of the Enemy Attacks (I reason that flip does not equal reveal as shown in the FAQ as reveal must be specific). However, p.23 of rulebook under "Forced and When Revealed Effects", last sentence: "When revealed effects do not reolve when the card is revealed as a shadow effect.", it seems that shadow effects are also revealed.

So what do other players think?

Hmm, I would say that you can't use A Test of Will because this is still a shadow effect. The shadow effect tells you to resolve the When Revealed effect, but that does not mean the card is "Revealed", it is still technically a shadow effect.

I would play it that I could not counter it with a Test of Will

I would agree with the others. First off, A Test of Will specifies that it has to be a card revealed from the encounter deck, I don't think that shadow cards are considered revealed from the encounter deck. Secondly, you'll notice that if you continue to read the When Revealed text on Sacked! that it says it cannot be canceled, so A Test of Will is useless even when Sacked is revealed as a normal treachery card. You would be better off using Hasty Stroke to cancel the shadow effect, which can be canceled.

Kiwina said:

Quite a bummer that Eleanor too uses the term "cancel" with regard to her ability vs "when revealed", looks like Miner of the Iron Hills or killing one of the unique Trolls are needed for Sacked.

Thanks for everyone who replied. I missed the "cannot be canceled" part in paranthesis when I post the OP.