Do you want to bring any of your NPC followers with you? In particular, you could bring Hozela, who originally belonged to the group of dragonborn into whose territory you are about to go. She's only a level 5 non-gestalt character, though, so she wouldn't be all that helpful in a fight.
As a reminder, the other NPC followers you have are:
Sorjla: a grig cleric loyal to Thorsaska, who is aiding you because Thoraska told her to.
Hathnor: An arborean fighter who is on less-than-friendly terms with the arborean leader (and everyone else in the Pyramid seems to be on unfriendly terms with the arboreans).
The three allies you just aquired: Harumon, the eladrin Ilyana and her brother Venali.
And three of the former PCs: Yessai, Mundarion, and De'Arv'Ki.
The NPCs aren't really strong enough to be very helpful in combat, so if that's all you'd want to bring them for you could probably just leave them behind.
You make your way back to the first floor, and head towards the dragonborns' area.
"I believe my final splinter is in the eastern-most chamber of Garash Vren's territory,"
Vyrellis explains as you go.
Before too long you reach a chamber which is guarded by a tall, slim, and armored dragonborn.
"Halt! Who are you and why are you here? Oh, it's you. You're the ones with whom Hozela left. Have you come back to tell my leader what you encountered in the Library of Whispers?"
"Indeed. And to discuss other things also. We continue to have many errands."
I'm fine with bringing Hozela along just to help diplomatically with the dragonborn, but I'm happy with it just being the three of us exploring any new territory.
The dragonborn leads you north into another chamber. A tall, muscular blue dragonborn greets you.
"Hello. I recognize one of you, but not the other two. I am Gharash Vren, the leader of this group. We have been trapped in the Pyramid since we were pulled in from Sigil."
"Did you get the Tome of Vecna's Breath?"
Reminder: Way before Jan or Krezen joined the group, the dragonborn fought and took hostage a group of harpies who were allied with the original PCs. Garash offered to release them if the PCs brought him the "Tome of Vecna's Breath" from the Library of Whispers (where a splinter of Karavakos was). He also sent Hozela to help the PCs get it. Garash Vren at the time didn't say anything about where a splinter of Vyrellis was. A few real-life years later, after Jan and Krezen joined the game, you did find a book that might be the Tome of Vecna's Breath in the library of whispers, after defeating the splinter of Karavakos.