Crystal flare

The crystal flare lights up the sky over the Rancour Bluff in a multi-coloured display. The lights dance and twirl above the mountains for a day and a half, visible across the whole of the Hollow Expanse.
The lights are seen as a sign from the ancestors to remember the joys of life. Because of this link, inbetween the flares, the tribes created the Shautane Grotto as a holy place to revere those who had gone before.

Festival

When the crystal flares, it is seen as a time for peace and festivity. Everyone joins together with those near them, and dances and celebrates the lights. Even during the time of the Nomadic tribes, when the different tribes frequently fought between themselves, these days were a time to lay down any weapons and compete with music instead.

Link to the phoenix

Unknown to the peoples of the area, the Radiant Phoenix is the cause of the flares. Their inate power builds up in the caves under the mountains. When it is time for the phoenix to rebirth, it flares up and out. When this occurs, anyone too close on the mountains is dazed and collapse.
Before the Formation of Fenixia, Horaeri, The First Emperor and his friends were in the Shautane Grotto when this occurs, so they were hit with the undiluted force of the phoenix. Instead of being dazed and collapsing, they were granted part of the powers to manipulate crystals.
Since then, the power has been split between the bird and the peoples of the Fenixian Empire, so the flares have been smaller and less dramatic. Many of the historical tales of this event are now seen as exagerated and untrustworthy.