As of today, Sagittarius has taken over the Zodiac Throne. But he appears to be playing a little bit of hide and seek with you!
It's probably a good thing that not many of the Zodiac families sit on the throne itself, that chair is too awesome to be sat upon all the time...
Standard disclaimer applies, if you do not immediately see Sagittarius where you think he's hiding, you may just need to wait 5-20 minutes for a cache update on your end.
In order to complete the main event plot, you will have needed to complete at least some of the storyline sections found in Grayview City. It is advised to do this first.