What killed the Prince of Persia series?
What did kill the Prince of Persia series?
Is the meaning the same in both questions? Are they both grammatical? Why or why not?
They are both good grammar, but they carry slightly different meanings.
The first question is just a request for information about what killed the series.
But the second one is worded in an unusual way.
According to grammaring.com, you can use a form of "do"/"did" with a bare infinitive verb for emphasis.
This version of the question emphasizes the reality of the "killing". It creates a contrast with some other thoughts the listener/reader might have. For example there might be many ideas for what killed the series, and the author wants to imply that most of the possible reasons did not kill the series; he wants to know what really did kill it.