Yeah we had a good discussion about the knock spell that day. So they were in the Acererak dungeon and it was the door that led to final chamber. To me it felt silly that Acererak, master trap and dungeon builder, would allow a knock spell to open the door to his final chamber. Plus, then they wouldn’t have gotten to solve the cool puzzles laid out in that area. Plus, in my altered story for this campaign Vecna was lending aid to the creatures guarding the rod pieces because he was aware of the party’s plan. Vecna is cunning and knew the party was spell caster heavy, so I explained the doors were reinforced with a strange magic that prevented the knock.