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?
Best Answer
Yes, you can cast a spell with any spell slot available to you
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:
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).