A small portal opens up wherever you strike, with a black mirror inside full of stars, ripping at all the matter in its immediate vicinity.   As part of a successful weapon attack or unarmed strike, you manifest a piercing blast of void energy from the target you struck, dealing 2d6 Force Damage in a 15x5 foot line, the direction of which is a straight line based on where you made the attack from. The damage increases by 1d6 Force Damage if the target is from another plane of existence.   All targets inside the 15x5 foot line must make a Constitution Saving Throw or become vulnerable to Cold damage for a duration of up to 1 minute, even if they are normally resistant to Cold damage. On the end of each of their turns, they must make another Constitution Saving Throw to end the effects early.