According to the Spirit of the Pass, Randas travelled to the Underground Pass beneath the Galarpos Mountains and was tempted by Zamorak. He apparently heard voices which turned into visions. He performed a ritual that would supposedly alter his spirit. He chanted some ancient words, and the spirit of Zamorak began to enter his body. Unable to bear the power of Zamorak, he lost his mind.
Randas still wanders around the Underground Pass. Lore says that when asked about Zamorak, he will relate his life story, and elaborate on the ritual he performed. Given the true nature of the pass, it is more than likely that the Dark Lord corrupted Randas, rather than Zamorak himself.
His journal can be found during the Underground Pass quest. Additionally, Koftik will read to the player near the beginning of the quest a section of Randas' journal that is not found in the book which the player can later pick up.