Hello.
I had killed Gambit's boss Voice of Riven twice and still don't get quest for Malfeasance.
As I get from guides it drop as soon as I kill this boss for first time. On first kill I had full inventory and when I didn't recieved it like my dudes I thought it was becouse I had no free space. But today I had killed it again and again do not get the quest. It's very frustrating and I couldn't find topics with the same issue. Everyone just says you recieve it when you kill that boss.
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The quest is actually a chance drop from the boss, you have just been unlucky so far.