Complex sentence: On September 20,2000, Eidos Interactive and Ion Storm announced in a press release that they would be releasing the software development kit (SDK), which included all the tools used to create the original game.
Split into simpler sentences: On September 20,2000, Eidos Interactive and Ion Storm announced in a press release that they would be releasing the software development kit (SDK). According to the announcement, the SDK includes all the tools used to create the original game.