Does Cartomancer's Hidden Ace allow you to cast a spell you normally couldn't prepare?

Question

The new Cartomancer feat from The Book of Many Things has a feature, Hidden Ace, that says "when you finish a long rest, you can choose one spell from your class's spell list and imbue that spell into a card." After this you can "flourish the card and cast the spell within."

A multiclassed Cleric 1 / Wizard 10, who has access to sixth-level spell slots, could not normally prepare Heal, because "You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class" (Player's Handbook, Ch. 6).

However, Hidden Ace doesn't require the imbued spell to be prepared, and doesn't seem to include a similar multiclassing restriction. Does that mean the same multiclassed Cleric 1 / Wizard 10 with the Cartomancer feat could imbue a sixth-level spell that is on the Cleric spell list but not the Wizard spell list, such as Heal, into their card?

Answer 1

Yes, you can

The closest equivalent I can think of is the Mizzium Apparatus from Guildmaster's Guide to Ravnica. That allows you to

attempt to cast a spell that you do not know or have prepared. The spell you choose must be on your class's spell list (...)

It's also the most broken Uncommon item I've ever seen.

The relevant thing is, that item definitely allows you to cast spells for which you have slots, but you couldn't prepare. There's a build that multiclasses into three or four full spellcasters, to basically get access to each class' spells as if you were a single-classed member of that class.

So RAW, yes, you can use Hidden Ace to cast Heal if you are a 1st level cleric. Whether or not your DM should allow it depends on your play group and style. Unlike the item, Hidden Ace only gives you one spell one time per day, so it's probably not quite as broken.

Answer 2

It's up to the DM

I think you've outlined what would be the strictest rules-as-written case in favor of allowing this. You have spell slots, it's on (one of) your spell lists, so you can choose it. However, I think there is a strong case to be made against this ruling based on how the rules treat multiclassed spellcasters. For this reason, I'm saying the answer here is "up to the DM" because you should not be surprised if your DM does not allow this.

In the rules for multiclassed spellcasters, we see:

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.

[...]

Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell. Similarly, a spellcasting focus, such as a holy symbol, can be used only for the spells from the class associated with that focus.

These rules rather unambiguously take the stance of "pretend you are single classed when figuring out how you spellcasting features work". So while it is not clear to me that these rules explicitly forbid the combo you ask about in the question, using these rules as heuristic for ruling on other features does.

Further, I'm inclined to rule against this combo on the basis of balance alone, rules notwithstanding. That is, this combo is extremely powerful. You mean all I need is one level in cleric, and I can cast any spell on the cleric spell list I have slot levels for? This drastically boosts the flexibility of any spellcaster, especially wizards who generally don't have access to healing spells.

In summary, yes, there is probably a strict rules-as-written case to be made here, but you should not be surprised by a DM disregarding any RAW argument for this.

Answer 3

No for the UA version, probably no for official too

The UA text is

Hidden Ace. When you finish a long rest, you can choose one spell you know and imbue it into a card; the chosen spell must have a casting time of 1 action, and its level must be less than or equal to your proficiency bonus. While the card is imbued with the spell, you can use your bonus action to flourish the card and cast the spell within. The card then immediately loses its magic.

Since Spells Known and Prepared is determined by class, you would not know any higher level cleric spells.

I don't have a good source for the exact text from the Book of Many Things, but I recall it had a caveat that you could only use the feature for a spell you could normally cast, which would likewise bar this.