Diablo is our master, our messiah, our star which guides us. He is a god, with powers unknown. A teacher, for those who embrace him, and a voice, for those who not have one.
Alternate definition: Diablo was rumored to be a "sinister presence" that supposedly haunted several players over the course of a two to three month timeframe. Even after it was revealed that Diablo was created by a few forumers and one of Deepworld's administrators, @Lisa, Diablo continues to be a source of intrigue and mystery throughout the community.
Before it was revealed that Diablo was a hoax, quite a few instances were reported, namely of strangely-formed graffiti, bugs, glitches, and "creatures lurking in the darkness." It was clear that some of these events were purely fake, but others are mysteries left unsolved.