Excelsior is an experimental drug used by the Freakshow.
The drug in question was an experimental physical booster program developed for the military. The compound warped and expanded their bodies over the period of just a few hours, giving them superhuman physical attributes, incredible pain tolerance and a powerful narcotic buzz. It also allowed them to expand their own abilities beyond the physical boost the serum provides. Cybernetic enhancements are a relatively new and untested innovation, and few people's bodies can handle the experimental limb and organ replacements. The regenerative properties the super serum provides allow Freakshow members' bodies to handle the implants with relative ease. Once the Freakshow realized this they wasted no time in "cybering up."
Normally a group like the original Freakshow would never have gotten access to the kind of power enhancing chemicals that have today become a staple of their daily diet nor the hardware and weaponry with which they festoon their bodies. Unfortunately for the rest of the city, fortune decided to smile upon the Freakshow one fateful night. The gang hijacked an armored truck transporting experimental drugs for Crey Industries. Never ones to shy away from trying new and potentially lethal drugs, some of the members decided to try out the truck's contents on themselves.