There are 10 Acharn Legend missions required for this achievement, all of which are available in the primary Story mode. They are very straightforward, stealth-based missions.
You can also do these whenever you'd like for the most part. A few require certain upgrades to be unlocked, but what I meant was that you can do these before you play the story or after you beat the game. I don't think it upgrades your dagger in any way other than aesthetically, so it does not matter when you finish them all.
However, be aware that they give a lot of XP per mission, which could unlock useful upgrades that
do make the game easier.
Also, to make these considerably easier you should go into them with a few Upgrades + Runes. For some of these you are allowed to use the bow, so I used the following Upgrades, which made the challenges not all that challenging:
AttributesShadow Strike/Lethal Shadow Strike: Allows you to basically teleport around a map. Without a doubt the most useful upgrades in the game.
Quick Draw: Combined with Shadow Strike, you can teleport around and get instant kills even while in combat.
Runes For The BowAscendant (Epic) - Reduces focus depletion by 50% (comes with the GOTY version).
Deadly Archer (Epic) - Increases headshot damage by 25% (comes with the GOTY version).
The following runes can be any level, but higher levels are better:Keen Eyed - Recover focus for stealth kills with the bow
Broken Mind - Chance to recover Elf-Shot with Drains or Brands
Sylvan Arrow - Chance to recover Focus for a headshot kill
All of those combined will basically make you an unstoppable, undetectable ghost. All of the challenges are doable without becoming unstoppable, but I played all of these Legends after I beat the game and had all of these upgrades, and breezed through them with minimal effort.
Here are the challenges you will need to complete. Credit for this list goes to
Zasta 360GameTV for thinking to transcribe all of the information
before completing the achievement unlike me.
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 Ghûl 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