To zero. The extra damage is triggered on a successful hit, not successfully dealing damage.
True, but it is extra damage, and it is +1d6, not 1d6. You're not dealing zero damage with your Trip, so why are you adding the +1d6 to zero?
Well, other than being algebraically correct?
The extra damage is not dependent upon there being any underlying damage at all. It is triggered by a successful hit.
If I agree to give you X(Baby Seals) + 1DN(Baby Seals), and X equals zero for any reason, I still owe you 1DN Baby Seals.
They are independent variables, not interdependent variables. If they were interdependent, the extra damage could not be dealt without the underlying damage, which would be expressed by a phrase like:
"...A (insert energy type) weapon deals an extra 1d6 points of cold damage on a successful hit if and only if the weapon does damage on a successful hit."
Besides, if we really want to play with semantics, PHB p134 says of Attack Rolls:
(emphasis mine)If your result equals or beats the target's Armor Class, you hit and deal damage.
Similar language on p135 says:
To score a hit that deals damage on your attack roll, your result must equal or exceed the target's Armor Class (AC)
So a successful hit, by definition, deals damage...even if it doesn't actually deal damage! It doesn't matter if that zero damage is because its a trip attack, or the damage is reduced to zero, or whatever- it still "damage" by the rules of the game. By RAW.
Of course, what they REALLY mean is that to have an attack have a successful result, you must have an attack roll that equals or exceeds the target's AC...in other words, you must hit your opponent. Once you hit your opponent, you may do damage...or trip, or disarm, or stun, or sunder... And, as I pointed out, the extra damage of energy enhanced weapons is triggered by a hit, not by dealing damage.
I'd say no SA damage when tripping- Sneak Attack damage is precision damage based upon the rogue's knowledge of anatomy and vitals, and applying that knowledge by striking those vital areas.
The cold damage, in contrast, is merely the result of an enchantment doing its job. No special knowledge needed, no requirement of exact placement...just a successful hit.
The rogue has the knowledge of vital areas, she's making an attack, her opponent has a discernible anatomy, and her opponent is denied his Dex bonus to AC. The 'exact placement' is taken care of by those factors.
So given that both are extra damage, and the requirements for both are met, why should one work and not the other?
PHB p50
Sneak Attack
If a rogue can catch an opponent when he is unable to defend himself...she can strike a vital spot for extra damage
Arguably, "legs" are not true vital spots. If they were, then logically SA damage should apply to anything with legs, including constructs and undead- the rogue would be messing up joints or breaking off chunks of her target's legs...
Vital spots would be vital organs or nerve clusters. For example, if I strike your vagus nerve correctly, I can disrupt your heartbeat, possibly your breathing, or perhaps you'll just go unconscious. But it is a smaaaaaaall target located along the jaw. Striking it takes skill and either precision or great force. The precision strike hits that spot and only that spot. The forceful strike is what we'd call a knockout blow- you hit the head hard enough and at the right angle to displace the jaw, driving the bone into the nerve.
Are there nerve clusters in the legs? Sure. And if I were to slash or punch them a certain way, I might make your leg go numb, causing you to fall.
But to strike that way, I must jab or slash a very small target- a very different kind of strike from wrapping a whip around your ankles to trip you up. For one thing, the force of the blow is distributed over a broader area of flesh and over a longer period of time (the time it takes to wrap around both legs).
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