Crossbow of Life/Death

As an enemy comes within 30 feet of you, you can chose to use your choice of necrotic or radiant damage, to summon a crossbow. this crossbow glows with one of two colors: purple if you chose necrotic, golden if you chose radiant. as you summon the crossbow, you pull out a bolt that is either purple or golden, (purple if you chose necrotic, golden if you chose radiant) and put it into the crossbow of life/death, and then you shot the bolt at your chosen monster, dealing 1d8 necrotic or radiant damage, your choice.