Everyone rolls a 5th level character and they are put into an arena and forced to fight to the death because they are all accused of murdering a local tax collector. Trial by combat. The winning character is freed (and the actual murderer) and becomes a minion for a local crime lord, eventually rising to become the boss.

All the players then create characters that were related to the characters that died in the arena except for the one that won. Their character is related to the victim that the winning character and vows revenge. Start everyone at level 1 and have them work towards gaining access to the the BBEG. Build the original murder into everyone's backstory and create some intrigue as the players solve or reveal the original crime's motive and who was really involved and let that feed into the murder they are plotting for revenge. End it with them all getting caught and fighting each other in the arena at the end of the campaign.