One with shadows
Does this legacy only trigger on "surprise attacks" or does it get checked anytime anyone initiates combat with you? And is it prime stat wisdom?

Balance of the sisters
Can you use exotic skills from your offhand using this legacy? would be nifty to see batter plus riposte hehe.

>One with shadows
> Does this legacy only trigger on "surprise attacks" or does
>it get checked anytime anyone initiates combat with you? And
>is it prime stat wisdom?

Surprise attacks. Yes, wisdom.

>Balance of the sisters
> Can you use exotic skills from your offhand using this
>legacy? would be nifty to see batter plus riposte hehe.

No.

What kinds of situations are deemed "surprise" attacks? For example, is it a surprise attack if you couldn't see your attacker before he initiated?

I've seen it fire on blackjack (prevented working), ambush (didn't prevent, just riposted) and strangle (prevented working). But I have no idea if it'll actually stop assassinate when it fires.

It's a number of specific skills generally associated with stealth or surprise attacks.

Yes, assassinate is included.