Really? You think that an ability that isn't stated to only exist in that adventure, and on a fairly weak variant of Vecna, isn't something that he would have somewhere else? Why do you assume that it's for that adventure only?
...who does that?? Totally bizarre.
in the adventure it said he knows if you cast anything and auto identfies what it is... but you are correct that is an added ability for the setting not in the stats (although I feel it should be)
I don't really like either of those but I see both come up on Reddit pretty frequently.
Literally never heard of that. Seems to render the whole system pointless, particularly with how easy going 5E us to start with. Being very, verybspecific about Components is pretty eventually in my experience.
I mean it depends on the spell. Command, for example, has only verbal components, and requires you to speak a one-word command, which the subject must obey if they fail their save. It makes total sense to me that a failed Command would come across as the caster just shouting “flee!” or whatever. There are probably other examples of spells that I could see not being obviously spells, especially when they fail.
Doesn't Command indicate that someone who makes their Save know that they had to make one? Or is that other Enchantment Spells?
Nope. Charm Person and Friends both specify that the target knows you used magic to influence them after they wear off but AFAIK the victims of charm spells generally aren’t aware that magic has been used on them if the spell fails.
I think that an Arcana check would apply, at least for a target that has Arcana proficiency.
Command and Suggestion are two of the spells that lead to the arguments coming up most often, because some groups think the verbal component is only the command or suggestion while others rule they're in addition to the "chanting of mystic words". Counterspell is more useful inn a group that thinks the latter.
