Being a high skilled assassin, you know exactly where to find the vital points of a creature's body. You use these vital points to do more damage and send the victims of your assassinations to the afterlife.
When you roll a 19 on a d20 for a weapon attack roll, it is considered a critical hit. In addition to doubling the damage as normal you also do an additional 1d6 necrotic damage. Any creature that is reduced to 0 HP by a critical hit done by you instantly dies. Any magic, skills or abilities that would trigger upon a creature's HP going to 0 don't take effect and the creature dies immediately. Only the divine intervention from a deity or the wish spell can negate it. Similarly, if a creature died like this, it can only be brought back to life by True Resurrection, Wish or if a deity brings it back to life personally.