D&D 5E You get one spell vs. a demon lord... which one?

[doctorbadwolf]

Spiritual Weapon is good.

As a DM, I’d have basically asked the player “healing, defense, backup, or offense?” And came in with the clutch save for the team based on the answer, or given him a one time boon based on the answer.

“Healing. If you or an ally fall in this battle, whether unconscious or undead, a spirit of healing will enter the fray, casting revivify on anyone who needs it, and Heal on everyone else.” Stuff like that.

 

[TaranTheWanderer]

I was going to say Banishment but they'd just make their save.

I like the idea of summoning the Solar...it's just very fitting.

 

[MarkB]

Modify Memory. Make him remember that he left the gas on at home.

 

[Eltab]

A special upcast Banishment that takes the demon lord and the PCs (but hopefully not the demon lord's allies) to its home plane for say one minute. If you can kill it before the clock expires ... it dies for keeps and does not reform or come back to haunt you.

 

[Umbran]

Holy aura was my first choice, but since we don't have the expensive tiny reliquary, it's not an option. Already asked the DM about that.

Why the heck does a god need a reliquary to enact a magical effect? What is more holy than the god itself!?!?

 

[Storyteller Hero]

Since it's a fighter, one particular spell that makes sense is Heroism. A level 8 Heroism would temporarily protect the whole party against being frightened as well as shore up everyone's hit points with temporary hit points gained per round.

The fighter, not knowing much about magic, prays for courage in the face of doom.

 

[andargor]

Reading that it seems your god is essentially casting wish. If your DM confirms this, then the effect of any spell of 8th or lower can be duplicated in 1 Action without components, giving you access to some pretty powerful spells against fiends such as Magic Circle, which normally would take 1 hour minute to cast, so you can combat drop it on him. Or a Glyph.

Honorable mention for Mind Blank, which basically makes 1 creature immune to mind-affecting effects, but it only affects one person.

EDIT: corrections

 

[Ancalagon]

Tiny Servant, duh. At level 8 you'd get 11 of them!

 

[Sword of Spirit]

A special upcast Banishment that takes the demon lord and the PCs (but hopefully not the demon lord's allies) to its home plane for say one minute. If you can kill it before the clock expires ... it dies for keeps and does not reform or come back to haunt you.

Most likely a demon lord will have taken precautions:

"Protected Essence. A powerful demon can take steps
to safeguard its life essence, using secret methods and
abyssal metals to create an amulet into which part of
that essence is ceded. If the demon's abyssal form is
ever destroyed, the amulet allows the fiend to reform at
a time and place of its choosing." - Monster Manual p.51

 

[Olrox17]

If the casting time can be waived like with a wish, then casting simulacrum on the demon lord might not be a bad idea.