At first I thought it was bad luck. No matter what level I play on in the Sewers of Arnoc Brutal difficulty I always get common grey items if it is an item drop. I occasionally get shards and characters as expected and the items have a higher chance of being higher star items if I finish with 3 stars than with less. But all items are ranging from 3 to 5 star and are grey common items. I have spent over 350 energy off and on for about a week now farming this level for a good Offense weapon and it is ALWAYS common. I have tried on both my phone and PC and it is the same. I mostly try the first stage as it is where you get the weapon but I've also tried the Boss stage and Stage 6 just to see and they too drop common grey items.