The Moon Bow eldritch invocation for warlocks (from Unearthed Arcana: Warlock & Wizard) allows you to go around without ever carrying a weapon or ammunition. You can summon your bow as an action, and then nock back the string and create an arrow instantly that you can fire. This is really, really cool.
You can create a longbow using your Pact of the Blade feature. When you draw back its string and fire, it creates an arrow of white wood, which vanishes after 1 minute. You have advantage on attack rolls against lycanthropes with the bow. When you hit a creature with it, you can expend a spell slot to deal an additional 2d8 radiant damage to the target per spell level.
However, would the bow created by this invocation be able to fire magical ammunition? For example, an arrow of dragon slaying? Or a +1 arrow?
Also, assuming you do use magical ammunition, can you expend spell slots for 2d8 damage per spell level on a hit, or can you only use this function with the white wood arrow created by this invocation?
Best Answer
As written, the bow creates its own ammunition "[w]hen you draw back its string and fire." As such, you cannot fire any other arrows you might happen to have on your person: the one the Moon Bow creates would get in the way.
That said, it would not be game breaking to allow the Moon Bow to fire magical arrows. Yes you will do extra damage but you are expending a magic item and a spell slot to do so.