Reactive Strike | ⮌

A Reactive Strike is a strike made in retaliation for a creature daring to move through another creature's space without asking (or Disengaging) first.
Simply put, you expend a Reaction | ⮌ to make an attack against the provoking creature, usually when the creature leaves your reach (not when they enter).
All creatures have a Reactive Strike by default, including players and enemies. Everything that would activate on an Attack does so on a Reactive Strike. Some feat/ures or abilities may alter, improve, or activate when using a Reactive Strike.