Bandages/Bag:

Image: https://i.imgur.com/7FZz1kP.png

Cause: Stacking of two different type of scars.

Metal Patch:

Image: https://i.imgur.com/ZUvxRB4.png

Cause: Melee (Sword, Charge, Execution, Battle-Hardened finisher, Counter, Shoulder Charge, Vault Kick, Last Chance Struggle), Stealth kill, Brutalize, Caragor, Ghûl, Arrow, Throwing Daggers, Fall, Shank, Debris, Slave, Crush, Graug Rock, Infight (Ranged).

Face Slashes:

Image: https://i.imgur.com/sE995mg.png

Cause: Melee (Sword, Charge, Execution, Battle-Hardened finisher, Counter, Shoulder Charge, Vault Kick, Last Chance Struggle), Stealth kill, Brutalize, Caragor maul, Graug, Ghûl, Infight (Melee, Ranged, Savage Weapon, Meat-shield), Fall, Shank, Debris, Slave, Crush.

Burn:

Image: https://i.imgur.com/u4QUWjg.png

Cause: Melee (Sword - Flame of Anor Rune, Counter- Burning Pitch Rune), Arrow (Explosive Demise Rune), Throwing Daggers (Light of the Valar Rune, Bright Fury Rune), Fire (Campfire, Detonation), Wraith Fire (Detonation, Fire Arrow, Fire Arrow - Fire Storm Rune, Wraith Burn), Wraith Flash, Grog Poison, Geyser, Ghûl spit, Infight (Flaming Weapon, Poison Weapon, Explosive Arrow).

Wraith:

Image: https://i.imgur.com/tuasKQS.png

Cause: Causes: Melee (Counter - Burning Pitch Rune), Arrow (Explosive Demise Rune), Throwing Daggers (Light of the Valar Rune, Bright Fury Rune), Wraith Fire (Detonation, Fire Arrow, Fire Arrow - Fire Storm Rune, Wraith Burn), Drain, Wraith Flash, Head Explosion (failed), Interrogate, Wraith Stun, Poison, Ghûl Spit, Infight (Poison Weapon), Dispatch (glitch).

Backstab:

Image: https://i.imgur.com/SRGc57U.png

Cause: Infight (Melee, Ranged, Meat-shield).

Notes:

• Backstab injury will disappear If the captain is Branded or if a different injury applies.
  
  
• Killing a Captain with blue fire/explosion can result in either Burn or Wraith scars.
  
  
• Sometimes, distinguishing characteristics (such as the Captain being covered in blood) will disappear if an injury stacks with another or Brand.
  
  
• A Captain killed during a Power Struggle will come back as long as he wasn't defeated by another Captain on a lower row.
  
  
• A Ranged Last Chance Finisher, A Caragor slashing from inside a cage or a Caragor breaking out of a cage permanently kill Captains.
  
  
• Bandages injury sometimes doesn't apply. The Captain will come back as he was after gaining the first injury.
  
  
• Fear and Hate traits might be gained by Captains that Return, depending on how they died.

Every Captain has a chance to come back if their head isn't cut off or exploded. The trigger I'm about to explain boosts that Return Chance to 100%. I will also explain when, how and where a Captain will return, as well as certain times the trigger will not work.

Tutorial:

Step 1: Choose an Uruk. You can use the trigger on any Uruk as long as they are not Warchief.

Step 2: To increase the chances of the Captain returning you must let him kill you. Once will still boost the chance of him coming back but doesn't "force" it. The magic number is 2. Let him kill you twice. This doesn't mean a Captain will only return if he killed you.

Step 3: Kill the Captain. Use the Causes list above and choose what kind of scar you want the Captain to have. Make sure he doesn't lose his head.

Step 4: "Cooldown". The Cooldown represents a 5 minute timer on which the Captain will not show up. It is only after the Cooldown that you can encounter him again. Simply roam around.

Step 5: "AI Rotations": Have you ever noticed Caragors randomly showing up? Uruks fading in while you are in Wraith World? A Worm spawning behind you 10 seconds after you looked there and there was no one before? This is what I call AI Rotations. AI (Uruks, Slaves, Beasts) leave the current area or simply despawn and make room for more to enter the area. Captain Rotations are a bit more specific. Returner spawns are the same as Ambusher spawns. I like to hang around in Warband Missions because there are Captain spawn points around those areas. To cause Rotations easily, Brutalize in front of large groups or kill Uruks.

Limitations:

If you are aiming for the Backstab injury, you must trigger Uruks killing that Captain however you can. If they die during Ambushes/Duels/Executions defeated by a lower Captain, they will be gone forever as the former is replaced immediately..

Warchiefs never come back, although a scarred Captain can eventually become Warchief.

The effectiveness of this trigger will decay overtime. It appears this happens after you get you 5th Returner. The trigger might not work every time after this.

Captains can die permanently. A Ranged Last Chance Finisher, caged Caragor, Caragor breaking out of cage and defeat by a lower Captain in a Power Struggle do not allow Captains to return.

Notes:

• A Captain that the game determined must come back will come back no matter what. If you advance time repeatedly, the spot of the "dead" Captain will never be taken by other Uruks.
  
  
• If you Fast Travel/Advance Time/Leave the area before identifying the Returner after he respawned, you might not be able to find him anywhere. He will only appear on Showdown missions as a Warchief's Bodyguard. His corpse will display any new scars on the Captain list. Check dead corpses on the list is a good way to know if a Captain is coming back in advance.
  
  
• If you wish to have many scarred Captains active you must first "override" the previous Returner with one more kill. Example, if you want a second returner while the first is still alive, you must let him kill you 3 times instead of the usual 2. This will work up to 5 scarred Captains in one region, one with each different scar (excluding Bandages).
  
  
• If you Brand a Returner and want another, you will only need to die twice to the new one, since a Branded Captain cannot be counted as a Nemesis.

Everything in this guide is from this Reddit post:

https://www.reddit.com/r/shadowofmordor/comments/4rni0w/updated_captain_injury_and_causes_list_how_to/