Black Flame Blade

You engulf a nonmagical weapon you hold with black flame, turning it into a magical weapon until the spell ends. For the duration, when it hits the weapon deals additional fire damage equal to half of the target's level or CR (whichever is applicable).
At Higher Levels.
When you cast this spell using a spell slot of 5th level or higher, you can maintain your concentration on the spell for up to 1 minute.