Warlock - bg3.wiki
The Arcane Trickster is a Rogue that has studied arcane magic like a Wizard, and they use spell slots the usual way. Unlike a Wizard, however, they don’t need to prep their spells ahead of casting.
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How do you feel about the Eldritch Knight?
The only way an Eldritch Knight would gain 9th-level spell slots is by multiclassing into a full caster class (wizard, druid, cleric, bard, sorcerer).