Diesalven was an arcanist of Quel'Thalas, who at some time in his life moved to live in Lordaeron. He became a trusted noble of the city, achieving lordship. There, he then began secretly practicing necromancy, eventually consuming him.
Having learned of Tirisfal Monastery's abbot planning to construct a runeblade empowered with the threatening strength of the Light, he amassed a force of mercenary bandits to siege the monastery, which eventually failed - resulting in the Swordbearer hiding the Sword within.
Official record of Diesalven's existence ended shortly after. He supposedly went to further practice his art of necromancy.
It is reported that, sometime later in life, he returned to the monastery alone to again attempt to prize the Sword, having learned of it's location. There he encountered Arellas, who took to protecting the Sword and engaged in magical eternal combat with Diesalven.