The ancient dragon god, sundered into two deities at the dawn of time, lives again in you. The only question is whether Bahamut and Tiamat will let you claim his power.
Prerequisite: 21st level, dragonborn
When a god dies, its power does not vanish. It can be consumed, often by whatever being was mighty enough to slay it. when Asmodeus killed the god he once served, for example, he claimed that power and made himself divine. When the King of Terror killed Io, the god's sundered body became Bahamut and Tiamat, and those god's claimed Io's power before Erek-Hus could steal it.
However, it is said the managment of Io's power remained free, neither absorbed by the new deities nor consumed by the primordial. Perhaps controlled by a shard of Io's mind, the divine power kept itself apart. Some say that one of his loyal angels harbored the fragment of his power, wile others claim it was embedded in one of his mortal servants-a dragonborn.
Somehow the shard of divine power has come to rest in you. For now, you are little more than a demigod or an avatar's distant echo of the dragon-god's full power. The more power you aquire, the stronger Io grows. Through you, Io plans to return and reclaim his mantle as the god of all dragonkind.