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Arkos was a powerful wizard that lived from 515 to 840. During his time, he created many powerful magical artifacts, some of the most notable being the Cambiaren Gaunlet and the Green Heart. He died in 841, stabbed in the neck by his lover, and thus weakened, even though he was technically imortal based on man's standards.
He was the last of the Valar to be destroyed by the dark master, and he was one of the lesser valar, and in his weakened state he was destroyed by the Dark Master an immortal greater valar, though disassociated with the valar.
His posessions including the Green Heart fell to the dark master, completing the dark master's defeat of the valar.