You have achieved such mastery over certain spells that you can cast them at will. Choose a 1st-level wizard spell and a 2nd-level wizard spell that are in your spellbook. You can cast those spells at their lowest level without expending a spell slot when you have them prepared. If you want to cast either spell at a higher level, you must expend a spell slot as normal.
By spending 8 hours in study, you can exchange one or both of the spells you chose for different spells of the same levels.
To take this feat, you must be a Wizard of at least level 10, and have your Intelligence score be at least 16 or higher.
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