Killer Instinct

For now, I think there are 53 possible items to get this achievement, and you must have 50 at the same time in your storage.

The item types are:

Common - 14
Rare - 14
Epic - 13
Killer - 12

I will list only the Killer* ones, since others, most people should have. If you have problems with others drop a comment and I will update.

- Stalker Transceiver, Wyvern-x and Security Codes - You can have those 3 at the same time, but you'll have to play the hatchery mission 2 times (excluding the one for the achievement). The last time you enter the hatchery, don't use the codes, just examine the incubators again. Just make sure you have six transceivers before entering the hatchery. Since you get them in pairs each playthrough. you need to start a new playthrough when you get four of them. If you don't have any of them, you have to get exactly 5 transceivers, enter the hatchery, get wyvern-x, give wyvern-x to the commander (getting the security codes), get exactly 6 transceivers (this is the important part), enter the hatchery again to get back the wyvern-x.
- Scroll of Restore, Greater Scroll of Restore and Superb Scroll Restore - these items come from packs or daily rewards, they also should spawn depending on the score you get after beating gargos
- Scroll of Ressurection and Phoenix Relic - can be crafted
- Familial Blood - this one you get from the quest Familal Blood playing as Mira against Maya
- Temperance & Vengeance - this one you get from the Temperance & Vengeance quest playing as Kan-ra against Maya or also in the Familial Blood quest after killing Maya (Thanks dieotze74).
- Ceremonial Dagger - drops from some common quest (I think it is cult of gargos, please let me know the name of it in the comments)

* - Besides those ones, there are also:

- Shadow Lord (blue) - drops from beating Gargos with a High Score on normal. This one I don't have, but I have confirmation of a score of 465k to get it (Thanks MMMDI , SaucyCooper & Yukeer).

- Shadow Lord (purple) - drops from beating Gargos with a High Score on normal. This one I also don't have. (Thanks SuperTy DX).

- Shadow Lord (gold) - drops from unknown. (Thanks https://www.trueachievements.com/DoobyDude23.htm).

Here is a video with the Killer Items I have:



Please let me know in the comments any corrections or items I'm missing

With the latest update today 11.22.16 it's looks like this will be easier to acquire because several items have been added to the game. So no longer will you have to rely on getting hatchery quest and doing multiple playthroughs.

So with the new update there is a Titan questline which you will follow. It doesn't really matter what characters you use not do I think the choices matter on prefight dialogue but I will provide you with how I got several new items. At the end of this questline you will have the option to recruit Eyedol which will definitely help against Gargos on godlike since he has immunity to gargos summons and a few of omen's moves.

This quest popped up on the 1st turn for me and the following quests on consecutive turns. Keep in mind after the 5th quest you will be required to craft an item consuming all 5 items that you received so only do this if you need 5 or less items. Even so with other items being added that you can craft should be able to get several new rare to killer items.

1st quest: Kan-Ra's request
Prefight dialogue I chose to help Kan-Ra post fight I received a Killer item called "Titans Soul"

2nd quest: The Titans hand against Kim wu
Preflight dialogue I chose to attain great power after the match I received The Titans hand 

3rd quest: The Titans eye against glacius
P refight dialogue I chose the stars after the fight I received titans eye

4th quest: The Titans club against aria
Preflight dialogue I chose time and after the fight I received The Titans club

5th quest: The Titans head against aganos prefight dialogue choice doesn't matter after the match I received The Titans head 

After this point you can craft shattered titan in emporium but will consume all items you just received but you need to craft in order to move forward with the next missions if you want to.

Quest 6: Awaken the titan against shadow jago refuse to give head to kan ra after the match I don't remember receiving any unique items after this match

Quest 7: The titan reborn against eyedol will receive moldy wolf heart and life ember 
 
Eyedol unleashed 
I didn't really get anything out of this match

You will face Eyedol 3 times after the last time you will have the option for him to join the Alliance.

Epic Embalmed lizard foot can be crafted
Common Moldy lizard foot
Epic embalmed fingernail can be crafted
Killer fusion tissue prototype can be crafted
Epic Elixir of greater luck(not sure how I received this) think it was against Aria in a shadowborn mission.

So in total I received 13 new items from this latest update. If you receive anything else not listed please let me know and I'll edit this giving credit of course.