I was wondering if anyone else used NPCs in the PC's assault on the fiery sanctum. I did this for Sunday's session, using 14th level Shensen, Fellian, Cora Lathenmire, and Annah Taskerhill. The party already had 6 members, but I felt that for a big, assaulting the BBEG's type adventure, some of the other heroes of cauldron would step up.
Unfortunately, this slowed down gameplay considerably. I ran two big battles in the last session. The first was Freija, 2 Kelubar, Dericini and 4 Flamewarders. The Second was Nulin, Grehlia, Ardeth, Ti'irok, and the other 4 Flamewarders. Including the summoned creatures in Freija's battle that's 10 Baddies, 6 PCs and 4 NPCs...20 total, 14 of which were controlled by me.
Cora Lathenmire was relatively ineffective. She couldn't get past the damage reduction of all the evil outsiders. She helped by flanking.
Shensen was a little more useful. Her Ice Storm was effective against the Flamewarders and Ti'irok. Her flamestrike was semi-useful against the evil outsiders. She also removed disease to get those that failed their nausea saves against the Kelubar.
Fellian used some of his spells to help bulstor the allies. His Pathwarden levels came in handy when they were in acid fog, and he could move unhindered.
Annah Taskerhill was by far the most effective. She used her greater dispel magic to remove an acid fog that hampered the party. She used greater invisibility on the alakast wielding rouge which allowed him to beat down the harankin. Her bardic music helped the group score a lot more hits and damage.
For next session, I think Shensen and Cora are going to depart for the surface. They have not been particularly effective, have taken a fair amount of damage, and Shensen has used almost all of her spells.
Annah Taskerhill only has a few spells remaining, but maintains a lot of her scrolls and 6 more uses of bardic music ability.
Fellian has a fair amount of his spells left and a stoneskin from Shensen.
I would definetly warn against using NPCs if you don't want to slow down gameplay A LOT. I did a fair amount of prep work (about 20 minutes an NPC) and their turns still took forever. Caveat DM.