The Brawler's mask
You've kept the mask of the Back Alley Brawler as a trophy. It's a reminder of the adventure you like to call:
Brawling with the Big Boys
It began with a contract on the Back Alley Brawler, issued by Arachnos to Crimson Revenant. The hero hunter knew whoever did the job would take a lot of heat from the Paragon City government, so he passed the job on to you. Of course, the Brawler would be well-guarded. To get to him, you first had to create a distraction. You journeyed to Sharkhead Isle, where you stole a number of Cage Consortium explosives.
Back in Nerva, the plot thickened. You took the explosives to the Longbow base intended to receive the the Brawler, and planted them. You then made your escape and triggered the explosives, leaving a blasted mess in your wake. The Back Alley Brawler's welcome was certain to be less secure now.
The Brawler arrived on Liberty Isle, but Longbow had not yet had time to muster a new security regiment. You swooped in and tore the hero to shreds. He now bides his time in an Arachnos prison, and you have risen in infamy both at home and in Paragon City. Not a bad day's work.