- knocking the target unconscious with a single well-placed blow while unaware or
- grappling the target in silence (covering the mouth, restricting breathing, whatever).
The grappling approach is far more likely to remain silent, at higher levels having a 100% chance of silence, but requires the user to defeat the target in a series of grapples. This may not be a viable path against some enemies. In those situations, a knock on the head may be a better option. Targets with higher constitution are significantly harder to incapacitate and those with higher intelligence are better at incapacitating. Levels in stealth also help to do it quietly.
Would this be one of the core abilities for your character? Better for staying unnoticed than backstab and without the pesky morality of murder. How would you use it? Let us know in the comments below.