Hello Mate,
I'm not exactly sure what you're asking by saying "Caesar's death official marks the end of?" because there are a number of things that Caesar's death marks the official end of... However, I'll try and help you out as best I can.
Julius Caesar died March 15th, 44 B.C. The biggest thing that ended after his assasentation was that the Roman Republic Ended. However Caesar left behind him a legacy that people still talk about today, and historians still study.