Elmofongo: BUT now I must face the Demon in the Dagger, but these cultist casts hold spells on my party, Strategic Recommendations?
Ring/potion of Freedom.
If you don't want any of your party to turn on you as ghouls, you must have some dispel arrows/spells/scrolls, because the Tanar'ri casts death gaze regularly, and any member stricken will slowly die, then turn into a ghoul. If you see them dying, dispel is your only hope -- potions don't work.
Tactics: you MUST concentrate on the head disciple first, because she's a spellcaster who will cause you grief, then concentrate on the remaining disciples -- ignore the Tanar'ri for now; do not bother with buffs, because the Tanar'ri also dispel those; summon, summon, summon, to keep the Tanar'ri busy while you deal with the disciples; when the last disciple is dead, concentrate on the demon. All of this is because when you kill it, the demon jumps into any live disciple.