Reviewing an answer to a recent question I posted, I had a thought about Armor of Agathys. In the event of a Sorcerer 18 / Warlock 2 build, would the character be able to cast Armor of Agathys with a 9th level spell slot, thus getting 45 Temporary Hit Points and dealing 45 damage when hit?
Is the spell in any way limited to being cast using Warlock spell slots or to a maximum of a 5th level spell slot?
As a multiclassed spellcaster you can use any spell slot you have to cast any spell that you know/can prepare from any of your spellcasting classes. There is no restriction on which slot can be used for which spell.
This is even specifically allowed in the PHB section on multiclassed spellcasting:
If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class [...] you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.
As a Sorcerer 18 / Warlock 2 you have one 9th level slot which you can use to cast any spell you know/can prepare which includes armor of agathys (a 1st level Warlock spell).
This is a multiclassing issue, so the following questions arise.
Yes. As Armor of Agathys is a 1st level Warlock spell, and you have 2 levels in the Warlock class, it is clear that you have this spell.
Spells Known and Prepared. You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class.
Yes. In your example, at Sorcerer level 17 you would get your 9th level spell slot.
You determine your available spell slots by adding together all your levels in the bard, cleric, druid, sorcerer, and wizard classes, half your levels (rounded down) in the paladin and ranger classes, and a third of your fighter or rogue levels (rounded down) if you have the Eldritch Knight or the Arcane Trickster feature. Use this total to determine your spell slots by consulting the Multiclass Spellcaster table.
Yes. It is explicitly stated that this is possible.
you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the Spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.
