Deckard Cain, also known as Cain the Elder, is the final living human in the line of Horadric mages. Appearing in all of the diablo 3 gold , Cain provides the player with invaluable advice for many quests, and it is he who initiates the Archbishop Lazarus and Diablo quests in Diablo I, as well as multiple quests in Diablo II. Cain, in both games, can identify items for a fee (or for free if rescued by the player in Diablo II), and serves as a town elder throughout the events of d3 goldHe is descended from Jered Cain, who captured Diablo two-hundred years before, and is schooled in the most obscure lore and history of the arcane. 
As of Diablo II, Cain is the only known survivor of Tristram's destruction, save the Dark Wanderer himself. Cain, rather than being maimed by the demons, was left in a cage near the well in the center of town to die. In Diablo II's Search for Cain quest, Cain is rescued by the player, and afterward follows the player through each of the game's Acts. Griswold, now a reanimated monstrosity, was corrupted by Andariel herself and attempts to prevent Cain's rescue. Once Cain has been rescued by the hero, he will identify items free of charge. If the player neglects to rescue Cain, the rogues will do so after the Sisters to the Slaughter quest and the player must pay 100 gold for each item identified, though he will still follow the player through each Act.
Deckard Cain returns in Diablo III. Cain's journal is published as lore. His journal acknowledges that while Cain dreamt of being the final of the Horadrim, he is in fact not. It tells the events of Diablo and Diablo II in an abbreviated form and suggests numerous times that Cain was an elder and lorekeeper from Tristram, without much prior knowledge of the outside world - a stark contrast to the well-versed and knowledgable "last of the Horadrim" loremaster of Diablo and Diablo II. It is also strongly implied that Cain had no previous knowledge of the Worldstone prior to the events of the Diablo II ending.
It is unknown whether Cain lied when he presented himself as "the last of the Horadrim", and his reasons for doing so, that he had given himself a self assuming title, instead of what would be his proper title 'Last of the Horadrim in Tristram' or suffers from (possibly age-induced) dementia or Alzheimer's. The fact that the Archangel Tyrael also refers to him as the "Last of the Horadrim" further clouds the issue and drives you to find more d3 items