A Rogue-aligned artifact (either a first gift or a crowning gift) that is some sort of small blade whose invoke effect allows you to teleport behind nearby enemies and attack them, perhaps with bonus backstab damage for a short time. Or more powerfully, you instantaneously warp behind and backstab several different enemies within a radius of 10 or 12 from you all at once.

Rogues gain a backstab-related bonus when they and their target are in darkness. Possibly, just make many/most attacks in darkness deal backstab damage when they ordinarily wouldn’t.

A more complex implementation would be to differentiate monsters that aren’t susceptible to darkness because they spawned in a naturally dark area from monsters that are.

If the player attacks a monster in melee immediately following a teleport (possibly only a spontaneous teleport), they get a backstab bonus on it, even if they’re not a Rogue.

Backstabbing can additionally occur if you are stealthy and the target monster does not correctly know where you are.

Rogues get backstab bonus against any incapacitated enemy (sleeping/paralyzed) in addition to fleeing ones.