You can unlock this achievement by completing all the Acharn Legend Missions.

There is a total of 10 missions.
Below is a list of names of the missions, objectives, and bonus objectives.

The Mithril Blade
Objective: Stealth Kill 10 Slavers in 3 Minutes
Bonus Objective: Avoid detection

Clear the Skies
Objective: Stealth Kill 10 Archers without setting off the Alarm
Bonus Objective: 5 Ledge Kill Uruks

The Wraith
Objective: Stealth Kill 5 Uruks without being detected
Bonus Objective: Kill 3 Uruks with Strike from Above

Deathly Whisper
Objective: Kill 5 Uruks while they investigate your Attract position
Bonus Objective: Avoid detection

Display of Power
Objective: Brutalize the Guard without setting off the Alarm
Bonus Objective: Stealth Kill 5 Uruks

The Darkness Within
Objective: Reach Executioners watch without being detected / Stealth Brand the Archers without being detected / Kill the Uruk Patrol
Bonus Objective: Kill no one until the ambush is set

A Knife in the Dark
Objective: Kill all Uruks without being detected
Bonus Objective: Stealth Brand 3 Defenders

The Scouring of the Shadow
Objective: Avoid detection and eliminate Uruk Teachbearers near each Ghl Nest
Bonus Objective: 5 Attract or Distract then kill Uruks

Fatal Secrets
Objective: Follow the Messenger and eliminate the Worms without being detected / Eliminate the Messenger
Bonus Objective: Kill 2 Uruks with Lethal Shadow Strike

Shadow and Steel
Objective: Stealth Kill 5 Defenders without raising the Alarm
Bonus Objective: Kill 3 Uruks with Strike from Above

---

Walkthrough -