Aidan

Prince Aidan is the oldest son of King Leoric, the father of Leah (with Adria the Witch), and the Dark Wanderer. Though Aidan is seen only in Diablo I and Diablo II, he was invented in Diablo III and metconed into existence in the earlier games.

Revised History

In Diablo I there was no Aidan. Prince Albrecht was King Leoric's only son, who was abducted by Archbishop Lazarus and taken into the depths of the Cathedral where Diablo's essence, still immaterial and largely trapped within his soulstone, took possession of Albrecht's body and mutated him into a host for the red-scaled Lord of Terror. This demon was defeated (in the official story) by the Warrior, who then attempted to contain Diablo's essence within himself, gradually lost his humanity as the Dark Wanderer, before ultimately transforming into Diablo's new host body. He was defeated in the final events of Diablo II.

In this revised history, Aiden impregnated Adria before fleeing Tristram. Her child was Leah, who Adria left with Cain before vanishing to the East herself, on her two-decade secret mission of evil.

This is now the canon story, though none of it was imagined at the time of Diablo I or Diablo II. Events in the first game especially take on a new tone considering these changes. In this metcon the warrior isn't just an anonymous hero battling demons; he is King Leoric's son, dealing with the evil aftermath of his father's madness and murder by his own guards. During the events of the game this re-imagined Aiden must defeat not just King Leoric, but his father, in reanimated bone form. Later, the search for the missing prince Albrecht becomes a desperate search for his own little brother, and the final battle against Diablo ends with the Lord of Terror fading away and leaving a human body... Prince Albrecht's!

Thus the metcon to turn the Warrior into Aidan forces the young man to battle his own father's demonic form and to kill his own younger brother.