I wish I had more options. His Dread Counterspell will work if he knows the target is casting a spell. The reason normal counterspell doesn't work against a spell affected by Subtle Spell isn't that Subtle Spell somehow prevents counterspell. Instead, it's that if you cast a spell with no verbal or somatic components, there may not be any "tell" that the caster is doing anything at all. At that point it's a purely mental action, and there's no reason Vecna would necessarily observe that any better than anyone else would.If, on the other hand, he clearly sees a spellcaster character pop out from behind cover and stand there without physically doing anything, you know for a fact that Vecna is smart enough to realize "someone is trying to cast a spell on me" and will use Dread Counterspell. Since there's nothing that says he can't use it repeatedly, he would lose nothing (except a reaction), though this could potentially be gamed--e.g. the Sorcerer pops out of cover and readies the action to cast a spell, waiting for Vecna to try to Dread Counterspell her, since it clearly has a trigger condition. Vecna tries and fails, because no spell has been cast. Sorcerer then fires off the readied spell, which cannot be subject to Dread Counterspell because Vecna is only allowed one reaction per turn and he wasted this turn's reaction on trying to counter a spell that wasn't actually cast.
So yeah. The problem is, Subtle Spell "prevents" Dread Counterspell in exactly the same way it "prevents" regular counterspell. That is, the person trying to counter the spell has to be able to know ("see," in the case of Dread Counterspell) that someone is casting a spell. That reason is normally seeing someone do the weird hand jives or hearing someone pronounce the mystic words, but logic might dictate that there are other reasons.
So...should I vote "no" because Subtle Spell doesn't OUTRIGHT prevent Dread Counterspell (due to not outright preventing regular counterspell either)? Or should I vote "yes" because Subtle Spell should have exactly the same effects on Dread Counterspell as it has on regular counterspell (namely, unless there's some other reason to know a spell is being cast, the countering person isn't tipped off and thus never gets the opportunity to try)?
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