Thieves' World sold extremely well, and carried Chaosium on its back for about a year and a half. It got placed into Glorantha, though the city's name was changed to Refuge. You can see it on Gloranthan maps - it's down at the southern edge of Heortland.
I ran a whole RuneQuest campaign in Thieves World. If you're familiar with the setting, one of the more menacing criminal groups were the Hawkmasks, who wore hawk half-masks to disguise their appearance. In my Gloranthan campaign, the plot background was that the Hawkmasks had been forced to go into hiding. But they were replaced by a group of villainous criminal ducks, who outfitted their minions with Duckmasks, which look just like what you think. Also, if you squeeze the hat's beak, it squeaks. A little extra touch my players hated. The Duckmasks of course were ostensibly enemies of the now-defunct Hawkmasks, and started seeking allies among Hawkmask foes.
Anyway, my players were hired by the Duckmasks as goons and thugs and went around doing some rather odd extortion and strongarm work. Eventually they found out that the ducks were actually working FOR the Hawkmasks, who had gone into hiding so their enemies would reveal themselves. The ducks weren't criminal masterminds at all - they just pretended to be. Eventually the real Hawkmasks returned and started wreaking bloody vengeance upon the revealed foes, who all thought they were working for the Duckmasks. Luckily for the players, they weren't really anti-Hawkmask, just easy muscle for the ducks to use, since they couldn't very well bully people themselves. (Art by Jenille Jaquays, done when she was still Paul. It's one of my favorite bits of Jaquays art.)