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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?