Backstab is a attack style through the Divinity series. This attack can only initiated with a small blade (typically a dagger or knife) from the target's rear.
In Divinity Original Sin 2, backstabbing is enabled via equipping a dagger or other small blade (e.g. a rapier). Outside of combat, the attacker needs to position themself behind the target then select the option to attack; instead of a sword being displayed, a hand holding a downward pointing dagger will be shown. Backstab attacks have a 100% critical hit chance.
In combat the targeting circle around a target will include an arced segment to the rear, this denotes the area where the attacker must stand inorder to initiate a backstab. When an attacker stands within the arc, only the arc is shown, not the full circle. The full circle is shown only when planning a character movement.