You attempt to stab your target in a particularly vital area, causing excess bleeding.
On a failed save, the target suffers from the Gaping Wounds condition for the next two turns. When affected with this condition, the target takes an additional 2 piercing damage from all in coming attacks. This effect can be removed earlier through healing.
On a successful save, you fail to stab the target deep enough and it does not gain the Gaping Wounds condition.
Save DC is 10 + STR or DEX mod + half of proficiency bonus. If you're able to make multiple attacks with the Attack action, this attack replaces one of them and deals normal weapon damage.