Transfusion

Make a melee spell attack against a creature you can reach that has blood in its body. On a hit, the creature takes 1d4 necrotic damage and you drain a number of vials of blood equal to the amount of necrotic damage dealt, which you may automatically store into an empty vial in your possession. If the number of vials of blood you drained equals or exceeds the target’s drain threshold (based on size as shown in the table below), the target must succeed on a Constitution saving throw or be stunned until the start of your next turn.

This spell does not change the number of vials of blood you can harvest from that creature’s carcass.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the damage increases by 1d4 each slot level above 2nd.