Spell Mastery is a Wizard feature gained at level 18 in the 2024 SRD. You have achieved such mastery over certain spells that you can cast them at will.
You have achieved such mastery over certain spells that you can cast them at will. Choose a level 1 and a level 2 spell in your spellbook that have a casting time of an action. You always have those spells prepared, and you can cast them at their lowest level without expending a spell slot. To cast either spell at a higher level, you must expend a spell slot.
Whenever you finish a Long Rest, you can study your spellbook and replace one of those spells with an eligible spell of the same level from the book.