I would think the general lore, for starters, and that he might have the Book of Vile Darkness, a powerful artifact.
As for features: he deals powerful necrotic damage (which is why Pyre has armor of necro-resistance), and he is difficult to cast a spell against (which Ezabard has learned firsthand as well), he likely has undead servants, and of course he has powerful magic in other respects. Ezabard has already seen several magical wards in this lair.
His other features (Flight of the Damned, Rotten Fate, etc.) are probably not known. After all, I can't imagine many creatures who've dealt with Vecna have lived through these features.
Within another round or two, probably other features will be quickly revealed.
I think more importantly is for you all as players to be mindful of what is happening to other PCs and the situation in general. I feel like somethings might be overlooked, but I could be wrong.
Metagaming, was there anything specifically you wanted to know about?