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.
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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.
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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
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Scroll of Ressurection and
Phoenix Relic - can be crafted
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Familial Blood - this one you get from the quest Familal Blood playing as Mira against Maya
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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).
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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:
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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).
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Shadow Lord (purple) - drops from beating Gargos with a High Score on normal. This one I also don't have. (Thanks
SuperTy DX).
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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