Ok so since this new Halloween theme is out I thought how about I try to find Xylar so me and 5 others went into Atheons raid wearing Atheons mask. Once we killed the "final" boss something strange happened the mask started to glow we all thought it was just a little thing included but we were wrong at the bottom of my screen the words "Xylar, The timeless has awoken!" It appeared at the back of the room behind the crystal flying towards us lets just say it looked awesome. We were not able to kill it as we couldn't figure out the sequence to the battle. So the requirements are to summon it are as followed: Atheon mask all fire needs to wear it.
Please tell me if someone else has found this I will happily give them credit.
[spoiler]Xylar was at 310*[/spoiler]