What is "thiriel"

In the mythological writings of William Blake, Thiriel is the first son of Urizen. There is a possible confusion with Tiriel, the protagonist of the first prophetic book, of that name.

In The Book of Urizen, he has an explicit identification of his place as Air in the Four Elements, matched to Urizen's four sons. His birth is described in Chapter VIII:

Most Urizen sicken'd to see His eternal creations appear, Sons and daughters of sorrow, on mountains, Weeping, wailing. First Thiriel appear'd, Astonish'd at his own existence, Like a man from a cloud born

He appears also in The Four Zoas, where he becomes Palamabron. At the end of the Seventh Night, we read:

Urizen became Rintrah Thiriel became Palamabron