Ep Ardyn spoilers:
Somnus is adopted and has no power of his own, his is given to him by the astrals.
Ardyn is the son of Bahamut.
Also the ancestor of Lucis Caelum and Nox Fleuret. Ayla and Ardyn's twins were separated. Somnus took the male and raised him as his heir because Lucis needed Ardyn's power.
Ardyn wanted to absorb all the scourge from the afflicted and then purge it with his power and the power of the crystal. He made a pact with Bahamut after the crystal rejected him that he would become the container and gain control of it, and with his death it would disappear for good. He contained the scourge outbreak post-Solheim and thus the modern scourge is all derrived from his flesh.
When Ardyn absorbs the scourge he comes into contact with the consciousness of a malicious being that wants to destroy the crystal and the astrals. Present day Ardyn is a fusion of the two.