Crawling Claw Vial

This quartz-inlaid vial contains a severed hand, engulfed in a grayish mist.  As an action, you can smash the vial on a creature within 5 ft. of you or throw the vial up to 20 ft, shatting it on impact.  In either case, make a ranged attack against a creature or object, treating the vial as an improvised weapon.  On a hit, the target takes (3d4) necrotic damage, and must succeed on a DC 10 Constitution saving throw or become poisoned until the end of its next turn.  If the saving throw fails by 5 or more, the creature falls prone.  A crawling claw appears as close to the target as possible, and attacks any creature closest to it.  Once it attacks one creature, that creature becomes its primary target.  The crawling claw appears with an amount of temporary hit points equal to half the amount of damage the vial dealt to the target (round up).  It also gains a +2 bonus to attack rolls.  When the crawling claw makes an attack roll, it can forgo the bonus and if the attack still hits, the crawling claw can add +2 to the damage roll.  Additionally, the crawling claw knows one necromancy cantrip randomly determined by the DM.