The sphere in the Supreme Council of Cui Fanke can be broken with the tools of eye, heart, brain and chain synthesis, and then you can enter the hate prison. Mephistopheles's prison is on the third floor. After killing Mephistopheles, you can enter the next chapter 4. As for the task reward, there is no Mephistopheles's soul stone. If it is broken, it will fall out of the Hellfire of the River of Fire in Chapter 4.
My remarks are for reference only. Please experience the details yourself! !