Assassin passive idea

For some assassin X: "First auto-attack that assassin X lands on a target records missing health of target. All subsequent spells and auto attacks over the next five seconds by assassin X deal bonus damage as a % of this recorded missing health."

A skill like this could help some currently weak assassins fulfill their role of eliminating squishy targets that have already been poked down without giving the assassin the power to 1-shot full-health targets.