When they arrived, an Aspect of Vecna was waiting for them. It wanted to bargain to get the Eye of Vecna back from the party invoker. (Backstory to this is
here.) The Eye is in the invoker's imp, placed there both to achieve a power up, and to stop Levistus (who placed the imp with the invoker) using the imp as a spy (by creating a Vecna-ish shield of secrecy). Unfortunately the party's conflict with Torog, as linked to above, had led to the invoker choosing the Raven Queen over Vecna as recipient of the souls of the Underdark's dead In retaliation, Vecna had used his control over the Eye to strike down the imp, which meant that the imp was currently lifeless (and hence the Eye inactive).
The bargaining was unsuccessful, however, as in an earlier session the invoker had already agreed to help the rest of the party try and destroy the Eye if they could find a way; and he now held to that agreement. The Aspect threatened a bit, but the PCs stood their ground and (recognising a superior force) it teleported away.
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the Aspect of Vecna reappeared bringing back up (undead cultists, lich vestiges and four demons under its control).
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In the first round the invoker dominated Vecna and made him dismiss one of his summoned demons. (I had described the demon appearing by means of gate. The player had his PC order Vecna to end the summoning. The established fictional positioning made this clearly feasible, and so it happened.) And then before Vecna's turn could come around again, the cleric-ranger stunned him with a reasonably newly acquired daily power. To add insult to injury, the chaos sorcerer rolled a 1, pushing Vecna 1 square. Vecna failed his save and went tumbling 100' to the ledges below the earthmote. Then something (I guess one of the demons?) hit the paladin and pushed him over the edge. At which point an Acrobatics roll was requested, to "do a Gandalf" (from the Two Towers film) and fall down on top of Vecna. The roll was successful, and the paladin dealt damage to Vecna with a successful basic attack, as well as taking damage himself for the fall.
While the other PCs cleaned up uptop, the paladin successfully solo-ed the now-bloodied Aspect, but (at the behest of the invoker) only knocked it unconscious (and then used his Marshal of Letherna daily utility to prevent any regeneration that might let it come back to consciousness). The invoker then came down and used an Undead Ward ritual, with the Aspect as a focus, to try and sever the connection between Vecna and his Eye. This was successful (between stats, feats and Sage of Ages the character has bonuses of around +40 to most of his ritual checks), so the imp came back to life, still powered up by the Eye but no longer subject to Vecna's influence. (But therefore once again able to send information to Levistus. When I chided the player for his PC not sticking the liberated eye in his own socket, his reply was that Malstaph (the PC) is not foolish enough to think that he's a god.)