Archwizard's Spellcraft

You command the most potent arcane magic and can
cast a spell of truly incredible power. You gain a single
10th-level spell slot and can prepare a spell in that slot
using arcane spellcasting. Unlike with other spell slots,
you don’t gain more 10th-level spells as you level up,
though you can take the Archwizard’s Might feat to gain
a second slot.