In the mythological writings of William Blake, Grodna is the third son of Urizen.
In Chapter VIII of The Book of Urizen his birth is described:
Grodna rent the deep earth, howling Amaz'd; his heavens immense cracks Like the ground parched with heat,[...]This shows his identification with the classical element Earth, in the alignment of Urizen's four sons.
References
Grodna (Blake) Wikipedia(Text) CC BY-SA