I guess the big question is, what is that pillar of darkness in the middle of the room? Is it literally a column of utter darkness?
This was covered a few pages ago:
EZABARD (ONLY): In the magical darkness, Ezabard sees a
tower of stone with what appears to be arrow slits (bold added). (RED ARROW)
Again, NO OTHER PC SEES THIS: all the rest just see the pillar of dark-flame.
Answer: Ezabard sees a stone tower with arrow slits
within the column of darkness (which is magical and preventing the others from seeing the stone tower). Your truesight reveals this much.
Assuming Vecna's counterspell needs line of sight, I should have seen him when he did his counterspell ability, even if he had been hiding before. I have truesight to counter darkness, illusions, and invisibility (is there anything that blocks truesight other than actual fog?). So I'm trying to think how else could Vecna be throwing down the whammy without me getting a glimpse of him?
What truesight does NOT reveal are creatures obscured or hidden from sight, nor does it allow you to see
through solid objects (such as the stone tower). IOW, Vecna is hidden from you, i.e. his Dexterity (Stealth) beat your passive perception (18). Until he attacks or casts a spell again, or you use your Action to try to see him within the tower and beat his Dexterity (Stealth) roll, he remains hidden. Truesight also does not reveal mechanical traps, secret doors, etc. unless they are also magical in some manner.
So, Ezabard would realize a few possibilities: Vecna is hidden from you, scrying you, or has assistance. However, since you know scrying is concentration, and so is flesh to stone, you think he must be hidden (probably within the tower to have Line of Sight) or has assistance.
Now, once you decide if Ezabard will run back into the hallway before the wall closes it off OR call for the others to join you, I can describe what you see briefly (if you run back) or in detail (if you are staying there and have a few moments to process what you see). The image I posted shows a LARGE room with the tower/darkness in the middle, the rest you can discern from the image as a player as you want, but what your character gets (in additional detail) must wait until you decide what Ezabard is doing in response to the closing trap.
This is what I am waiting on, so I can finish your turn depending on your choice.
Given that you have 20 Int and Truesight, I would encourage you to stop thinking about it as a D&D problem, and wonder why you couldn't see someone in real life
This is my preference as well. I am running this as an encounter, so I prefer players to react from the information given without me having to explain the mechanics/rulings behind it. But for the sake of moving things along...
I'm having trouble figuring out what's going on here myself. I can't imagine how I set off a trap
You didn't.
so I can only assume that Vecna cast a petrify spell on me while I was working on the trap
Vecna or an ally of his would be a reasonable assumption for Pyre.
So he's somewhere within range of that.
Probably in that black font in the middle, but I don't know why truesight can't see in there.
Because it is a stone tower and truesight cannot see through walls.