Spell Substitution on already-spent spell slots


Rules Discussion


New player here. Spell Substitution lets a wizard with that thesis take one of there spell slots and replace its prepared spell with a different one. Does this work even if the slot has already been spent to cast a spell? Because that would mean I could cast Mage Armor, replace that spell, then use Drain Bonded Item to reuse the same slot for a different spell.


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I think under a strict reading, it wouldn't work - not because you couldn't do the substitution, but because Drain Bonded Item specifies that you can cast a spell that you prepared and already cast. Not that you can cast a spell from a spent spell slot. So you have to have actually cast that spell in order to use Drain Bonded Item to cast it again.

So if you didn't prepare and cast Burning Hands, replacing your spent spell slot of Mage Armor with Burning Hands won't let it qualify for Drain Bonded Item.

That said, I am not sure why this would be too OP to allow.

Dark Archive

To break out Breithauptclan's answer a bit more.

You can indeed use Spell Substitution to replace the spell that was prepared in an expended spell slot. This option is called out in the 18th level feat Reprepare Spell.

Reprepare Spell wrote:
You’ve discovered how to reuse some of your spell slots over and over. You can spend 10 minutes to prepare a spell that you already cast today, regaining access to that spell slot. The spell must be of 4th level or lower and one that does not have a duration. You can reprepare a spell in this way even if you’ve already reprepared that spell previously in the same day. If you have the spell substitution arcane thesis, you can instead prepare a different spell in an expended slot, as long as the new spell doesn’t have a duration. Once you’ve reprepared a spell in that slot even once, you can use your arcane thesis to substitute only spells without durations into that spell slot.

In the above, the feat is calling out its impact on just this use case!

However, as Breithauptclan also said, it doesn't work with Drain Bonded Item.

Drain Bonded Item doesn't actually interact with your spell slots at all. It just allows you to cast a spell again which meets its conditions.

Drain Bonded Item wrote:
You expend the power stored in your bonded item, as long as the item is on your person. During your turn, you gain the ability to cast one spell you prepared today and already cast, without spending a spell slot. You must still Cast the Spell and meet the spell's other requirements.

So as long as you have:

1) Prepared that Spell today
2) Already cast that spell

You can use Drain Bonded item to cast it again.

There is some room to quibble here on its interaction Spell Substitution in that, if you no longer have the spell prepared, does it meet the first condition. For example, if I cast burning hands, but change the spell slot so I no longer have burning hands prepared at all, can I still use Drain Bonded Item for it?

Personally I would say the answer is yes, just as long as you haven't had another period of Daily Preparation since you last it. As you have met both of the conditions that Drain Bonded Item has called out.


Old_Man_Robot wrote:

There is some room to quibble here on its interaction Spell Substitution in that, if you no longer have the spell prepared, does it meet the first condition. For example, if I cast burning hands, but change the spell slot so I no longer have burning hands prepared at all, can I still use Drain Bonded Item for it?

Personally I would say the answer is yes, just as long as you haven't had another period of Daily Preparation since you last it. As you have met both of the conditions that Drain Bonded Item has called out.

Yeah, I could go for that as well.


So expended spell slots can technically be reprepared, but that doesn't enable me to use Drain Bonded Item to cast the new spell. That makes sense from a lore standpoint, too, since the spell wasn't prepared when I placed my magical power in the item in the first place. Thanks.

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