This question already has an answer here:

The Enlightened Magic feature of the Book of Exalted Deeds allows for 10th level spell slots per this question

Nothing seems to disallow level 10 spell slots, but would the spells cast with these slots assume level 10? There are two relevant rules as far as I'm aware. The section on Casting a Spell at a Higher Level says:

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. 

However, the section Spell Level says:

Every spell has a level from 0 to 9. 

These seem to contradict if the case of a 10th level spell slot with Enlightened Magic would allow for level 10 spells considering the definition of spell level.

Since Spell Level is defined between 0 and 9, what happens to a spell's level if you cast a spell with a 10th level spell slot?

marked as duplicate by Nick Brown, Conduit, NautArch dnd-5e Mar 29 at 20:51

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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    I don't understand how this question is different than the one you linked, and thus how it is not a dupe. – goodguy5 Mar 29 at 13:47
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    @goodguy5 the other question is about the usage of a 10 level slot. not the changing to a 10 level spell – David Coffron Mar 29 at 14:31
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    @T.J.L. this is not a duplicate as the question is about an entirely different rule. Spell slots and spell level are not the same necessarily, or if they are fypov then an answer could state that. – David Coffron Mar 29 at 14:37
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    I think this is a very close call, but the fact that spell slots are what was specifically asked about in the last question. Neither the question as asked or the answer as given make the link to the level of the spell cast. That link may seem obvious to people, but that doesn't make this a duplicate IMO. I have voted to reopen. – Rubiksmoose Mar 29 at 15:05
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    @NickBrown my question is "does the limit of spells being level 0-9 apply to 10th level spell slots?" – David Coffron Mar 29 at 16:06

Yes, a spell assumes the level of whatever slot is used to cast it

As you quoted:

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting.

So, if you have something that gives you the ability to cast using 10th level spell slots, then the spells cast using that ability would be considered 10 level spells for that casting.

And, as your previous question addressed, Book of Exalted Deeds does seem to allow the ability to do so when it says:

any spell slot you expend to cast a cleric or paladin spell counts as a spell slot of one level higher.

In this case the fact that the book says:

Every spell has a level from 0 to 9.

is overridden by the fact that the artifact in question allows the spells to be cast using slots higher than that.

As stated in Xanathar's Guide to Everything:

The game also includes elements — class features, spells, magic items, monster abilities, and the like — that sometimes contradict a general rule. When an exception and a general rule disagree, the exception wins. 

So, specific over general, the artifact takes precedent.

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