Innate technique: ratio

When landing a critical hit with a melee weapon you turn your opponents body into a weak point, causing any attacks made on the recipient to have advantage, and any saving throws made by the recipient have disadvantage for 1 round. Additionally the attack that deals this effect and any successive attacks made with a melee weapon for its duration deal an extra 1 damage dice of it's type or necrotic.

For this feat a 19 counts as a crit.