Well, there are very few things that might make a Kindred Prince willingly vacate his territory. Gehenna just happens to be one of those things. Tristan is getting the hell out of dodge at the advice of a vampire long thought mythical at best.
Interesting note - that actually is a word for word passage from the Book of Nod. It just actually happens to align pretty well the RP in TN in the past years. Eerie, that.
Also of note...this is a reason for the Kindred to flee to NYC. It can be the End of LA if people so desire, or others can come up with their own fandom reasons for heading out. I'm open to either or! :) I am, of course, available to ask questions if you have them. If you want info on Gehenna, here's a quick and easy primer that isn't as tl;dr as my post:
http://whitewolf.wikia.com/wiki/Gehenna_%28event%29 (P.S. I'm relatively certain that if the sun shines black or the moon shines blood, it will be Catherine's fault. That at least probably won't be Tristan. :P)