Void Piercer

The Void Piercer is a black crossbow made out of an unknown substance to the people of Eratros. It is smooth to the touch and feels like rock. The crossbow has a glossy finish and is almost entirely black. In fact the crossbow is of a shade of black so dark that it feels as if you are staring into a void. The black finish is broken up by the occasional mote of purple light that floats across the surface giving the crossbow a sense of depth. 
You gain a +2 bonus to attack and damage rolls made with this magic weapon. When you roll a 20 on an attack roll made with this weapon, the axe creates an instantaneous temporal rift at the target's location, which appears as a brief void in space. The target must make a Strength saving throw with a DC equal to 8 + the user's proficiency bonus + the user's Dexterity modifier. On a failed save, the target is warped by the rift, taking 6d6 force damage and suffering the effects of the slow spell until the end of your next turn. On a successful save, the target isn't warped by the rift, taking half as much force damage and suffering no other effects. Regardless of whether the target succeeds or fails, the rift then immediately closes, and the void disappears.
The Void Piercer produces ammunition whenever a bolt is needed to be fired. The ammunition is known as Void Bolts. Void Bolts are pitch dark pieces of ammunition composed of densely packed shadows. A creature hit by this magic ammunition takes psychic damage instead of piercing damage. This ammunition makes no sound when it's fired or strikes a surface. It leaves no discernible wounds and vanishes immediately after hitting a creature.