As the title says basically, one character in my campaign will receive a massive (but temporary) power boost. I want to have a custom subclass that adds to the class they currently are as this event may happen again, this way I can swap the character's subclass back and forth as need be. The character is an eldritch knight which can only cast spells to a certain level. Is there any way to edit the class so that it can cast cast level 9 spells?
You could add a subclass feature available at their current level that gives access to a ninth-level wizard spell, by adding a Spell with level 9, Wizard class spell list, and possibly a limited number of castings per rest? But you wouldn't be able to give the character 9th-level spell slots.
That said, I discourage you from doing this as a subclass; because the Eldritch Knight, as a Fighter, only gets spellcasting when you take the subclass, and switching it to a different subclass (even one that also has spellcasting) will ERASE their current spell selections. (Believe me. I found this out the hard way with my tweaked Arcane Trickster subclass.) You'd be better off implementing the power boost as a Feat instead, giving a Spell like I suggested (level 9 Wizard list, limited castings, no spell slots), and removing the Feat from the character when they don't get the boost.
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Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)
As the title says basically, one character in my campaign will receive a massive (but temporary) power boost. I want to have a custom subclass that adds to the class they currently are as this event may happen again, this way I can swap the character's subclass back and forth as need be. The character is an eldritch knight which can only cast spells to a certain level. Is there any way to edit the class so that it can cast cast level 9 spells?
I'm in your walls.
You could add a subclass feature available at their current level that gives access to a ninth-level wizard spell, by adding a Spell with level 9, Wizard class spell list, and possibly a limited number of castings per rest? But you wouldn't be able to give the character 9th-level spell slots.
That said, I discourage you from doing this as a subclass; because the Eldritch Knight, as a Fighter, only gets spellcasting when you take the subclass, and switching it to a different subclass (even one that also has spellcasting) will ERASE their current spell selections. (Believe me. I found this out the hard way with my tweaked Arcane Trickster subclass.) You'd be better off implementing the power boost as a Feat instead, giving a Spell like I suggested (level 9 Wizard list, limited castings, no spell slots), and removing the Feat from the character when they don't get the boost.
Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)
That seems like a much better idea actually, cheers for the tip.
I'm in your walls.