Chaotic Void-Javelin

A thrown Javelin takes on the dark chaotic energy of the Warlock's patron and travels 60-foot in a straight line. The Javelin absorbs all light within a 10ft range as it travels (see 'Darkness'). Each creature that comes within the 10ft range of the Javelin must make a Constitution saving throw. On a failed save, a creature takes 12d8 force damage and is knocked prone. On a successful save, it takes half as much damage and isn't knocked prone.

The Javelin remains for one turn after being cast, but cannot be reused. The 10ft sphere of Darkness remains, but deals no additional damage after the initial throw. 

The caster must make a Constitution Saving throw (DC 15) or take 1d6 damage.