Me and tragos just killed all the other Omega bosses and we were about to kill Omega Mephisto when all of a sudden he disappeared from the scene. We looked around for about 10 minutes but no hope. Is there a way we can get the Blue Worldstone Crystal or what's going on?

Here are some screenshots:

http://postimg.org/image/qc5e1omwz/
http://postimg.org/image/momxgtze1/