Lachdanan was captain of King Leoric's knights during the latter's reign as King of Khanduras. He was a righteous man who served his king dutifully. When Diablo tried to get a hold over Leoric's mind in order to possess him, the Arch Bishop Lazarus feared that the knight would become a problem. Lachdanan was viewed by all as a just and honorable man, and if he were to suspect something was amiss with Leoric there was a good chance the remainder of the court would believe him. Fearing this, the Arch Bishop accused the loyal knight of treason, and much to his dismay the accusation was pushed aside as outlandish.
As Diablo's hold tightened over the king, Lachdanan began to take notice of his king's errant behavior. As a last resort to remove prying eyes from Tristram, Lazarus convinced Leoric that Westmarch, the neighbouring country to the west, was planning to attack them. Lachdanan tried to reason with his king not to declare war, but Leoric was already at this point lost to madness and Lachdanan, honor-bound to serve, dutifully marched to war. The ploy sent both Lachdanan and most of the troublesome court far away from the king. The resulting war decimated the army of Khanduras against their militarily superior enemy, and Lachdanan was forced to retreat. During his time away, Diablo had extended his grip on Leoric and there was little left of his past self. Witnessing the terrible state of his beloved king, but unaware of Diablo, he killed Leoric.
Though he tried to honor his king with a proper burial, Leoric rose as the Skeleton King and both attacked and cursed his former servants. Many of his knights, like Gorash, fell victim to the curse and became servants of Diablo. Lachdanan alone managed to flee the burial chamber, but gripped by the same curse Leoric cast upon all of his former knights, he dared not return to Tristam. Instead he ventured deeper into the labyrinth under the Cathedral to die there.
During the events of Diablo I, Aidan encountered Lachdanan below the Cathedral. The details of this encounter are not entirely canon anymore, but the knight had run from Tristram and did not want to die cursed. He asked of Aidan to find a Golden Elixir, which he had heard could rid him of the curse. Aidan eventually found it, and thus helped save Lachdanan.