Posing as [[Dialon]], the Spirit of Determintaion, Diablo secretly led a group of religious fanatics in attempt to establish control over the realm. After the plan failed, the Tyrael created the [[Soulstone|Soulstones]] and charged the [[Horadrim|Horadric Order]] with the imprisonment of the Prime Evils.

Diablo was imprisoned in a Horadric monastery that later became [[Tristram]], and immediately started trying to spread terror and corruption. A few hundred years later, he was strong enough to try and corrupt [[Leoric|King Leoric]]. The Lord of Terror invaded Leoric's dreams, attempting to corrupt and possess the King. Leoric was able to resist Diablo's presence, but he was driven mad by the ordeal.