This will unlock through natural gameplay after completing any Dagger Mission found around the map (they are indicated on your map by, you guessed it, dagger icons - which have the blade pointed up). You can get this almost immediately upon starting the game, after unlocking free roam.
The first Dagger Mission you'll likely complete is "The Mithril Blade" which is in the Black Gate area. This particular mission requires you to stealth kill 10 uruk slavers in 3:00 minutes, with bonus points available for remaining undetected the entire time. Complete this (or any other Dagger Mission) and the achievement is yours!