On August 9th, 2012 a Steam Workshop announcement about Respecting Intellectual Property was posted. The article links to the stolen item that caused the issue, which turns out to be Marchutan's Blessed Mace from Aion. It's also discussed by the community on reddit where the mace is shown to be changed from the original in that it's green.
The Mace was being used as a community-created weapon within Dota 2 called Timebreaker and 24,603 people bought keys and got the mace through opening Sithil’s Summer Chest. All users that bought the weapon have now had it replaced with an alternative weapon and the user who made the mace is now banned and will lose out on any money he could have made from those sales.
It's a shame this happened. If the user that stole the mace wanted to see it again, he could have just played Aion.