Hello!
So, I have a few Leviathan completions under my belt (not enough to brag about though), so I've gotten the shader a few times...I can even get the armor from my collections.
Yet, I have no shader in said collections. I'm really not sure what I'm supposed to do, if the game wants me to do the raid again?
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In order to have the shader, you would have had to break down a piece of equipment that would have given you the shader unfortunately. Just completing the raid and having items with the shader on it will not give it to you.