Tome of a Lorekeeper

The pyrilen and gelidran were once a unified creature -- a pure riftseeker, neither male nor female, with magical powers that were nearly incomprehensible.

The god of the riftseekers, Bealu, was envious of their power, believing it rivaled his own and so he chose to ruin his own creation.
The magic and essences of the true riftseekers were split into the powers of fire and ice.

Bealu found if he kept the two creatures created out of the true riftseeker -- the pyrilen and the gelidran -- separate, he could control their power.

But when the two creatures were brought together to conjure magic, they could create the same intense power that caused the true riftseekers to be destroyed in the beginning.

Source: Ingame book