In what has to be one of the neatest stories I've read in awhile, this post on Reddit outlines the history behind the name of Diablo's most famous wizened sage, Deckard Cain. In short, the name comes from that of a real person - a 16-year-old boy, no less - whose father submitted it when he was just a baby for a contest that Blizzard Entertainment hosted back in 1996:
My name is Deckard Cain Elder, and I am 16 years old. When I was just a baby, Blizzard had a surprise drawing for Diablo with very vague details. All anyone knew was that if they wanted to be a part of the drawing, they had to send a postcard to Blizzard with their name on it, and after a said date, they would pick a card at random to win the prize. Being a very big Blizzard fan, my father decided to take part in this. He thought about it for a while, and decided that since my name is so unique, he would send the postcard in with my name on it instead of his. My first name comes from the Harrison Ford's character in the movie Blade Runner, my middle name is my mother's maiden name, and my last name is my fathers family name. Very cool. Pictures of the original contest congratulatory letter (signed by Bill Roper himself) and insanely awesome should-be-on-my-office-desk Diablo statue can be found here.
It had been about 4 or 5 months after my father had sent in this postcard with my name on it to Blizzard, and he had begun to forget about it, and accepted that, because of the period of time since he had sent the card in, we hadn't won this unknown prize. About 6 months after the drawing, a box arrived at my parents apartment in South Dakota. Not having ordered anything, they were curious as to why it had shown up. When they opened the box there were five things inside. A long yellow envelope, a copy of Diablo, a black hat with Blizzard in blue printed on the front, a very large Diablo T-shirt, and a blue statue of Diablo that stood a little over a foot tall. Inside the envelope was a letter from Bill Roper, the producer of Diablo, that explained that I had been chosen for the drawing prize. (I will have an imgur link to pictures of the Diablo statue, me holding the letter, and the letter up close for you to read.) The letter basically explained that the prize for being chosen in this drawing, was to have your name in the game. I honestly feel that, because of my unique name, Blizzard saw my name on the postcard and thought it suited the character they were looking to name. But either way, it's seriously cool. So if you ever wondered how Blizzard named Deckard Cain, you now know.