I didn't complete the Iron Banner quest line while it was here last time, but its not in my inventory. I also can't obtain the quest again from Saladin. I know I didn't complete the quest because I don't have the new Iron Banner shader and would really like to complete it this time around.
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Hey there. This season's IB quest is a short intro quest and there are no real rewards (you mentioned a shader, the only new IB shader is on Saladin's second set of rewards post-reset). All it does is unlock Saladin's rank-up rewards, if you can access this then you completed the quest, and may have even done so without realising.