The season 11 finale of "Bones" revealed the shocking twist that Zack (Eric Millegan) had managed to escape the psych ward and that he might even be the Peppeteer serial killer. Speaking to TV Line, executive producer Jonathan Collier explains that Zack is not the only character coming back for the show's final season. 

"We have a huge return coming up," Collier explained when asked if Millegan was the only character making a return for season 12. "My advice is pay attention to the first two, even three seasons. The seeds have been sown, and now we are reaping the harvest."

Collier did not directly confirm that Zack was indeed the Puppeteer but he did state that Zack had proven to have special abilities and that the writers have been inspired by real world events where people were able to take control of events despite being in circumstances where they normally shouldn't. This may be a hint of how Zack manages to get in and out of the psych ward.

The executive producer continued to explain that Zack's return had been in discussion ever since the team had been working on season 11. This concept became more realistic when they discovered that the upcoming twelfth season was indeed going to be the show's last.

It is further stated that Booth (David Boreanaz) and Brennan (Emily Deschanel) will have different opinions regarding Zack and that they will be on opposite ends in regards to judging or blaming Zack for his previous and ongoing actions. How this will affect their actions to catch Zack and the mysterious returning character have yet to be seen.

In a report from Crossmap, it is also confirmed that the season 12 finale will be a big 2-hour event. An official premiere date has not yet been announced although the report claims the show is likely going to premiere by 2017.