Tor Gakhul's Ring of String

With the snap of a finger, in a 50ft radius centered on the caster, razor sharp string laced with a deadly necrotic magic bombard the area in calculated and sneaky precision. Upon casting, any creature within range must make a Dexterity saving throw or take 9d6 slashing damage, or half as much on a success. The string stays in place for the duration, acting like an invisible and deadly trap; Any creature that remains within the ring will be sapped of their vitality, taking 3d6 necrotic on that creature's turn for as long as they remain within the ring and haven't moved during their last turn. If a creature moves within the ring, the sharp strings react to it and slash at the movement; Any creature that begins or ends it's movement in the ring must succeed on a Dexterity saving throw or take 10d6 slashing damage, and half as much on a success.