The zompig turned out to be a creation of the Firebirds who had assembled a bunch of them to take out the mages in the city. I don’t think we found out how this one got loose, but Artyom jumped in to save some poor woman from it and got messed up bad. We picked him up from the hospital later, then investigated and took out the people responsible.
The Firebirds are an underground organisation dedicated to toppling the privileged mage class and instituting a communist revolution. Lydia is a member, though not approving of this cell’s violent methods.