This deck was submitted by Simon Betz, and when he wrote about a Demon Tribal deck, I wasn't optimistic—after all, what's the point of making a tribal deck if there aren't any cards that reward you for sticking to that tribe?
Well, it turned out his deck was doing some cool things.
He'd been trying to take advantage of Master of Cruelties, a card that had captured the imaginations of more than a few readers, and realized that Master of Cruelties doesn't actually need power to reduce an opponent to 1 life (although it's certainly worse against creatures if it doesn't have any), so he could use Heartless Summoning to play Master of Cruelties, since he was planning to make it unblockable with Rogue's Passage anyway.
And once you're a black-red deck with Heartless Summoning, you really can't do anything better than cast more Demons anyway.
This is especially potent with Sire of Insanity, which can come out on turn four with a Heartless Summoning to make your opponent discard his or her entire hand—but your opponent probably didn't need that, right?
Below is the list Simon sent. Personally, I'd cut Rakdos Guildgate for a Swamp, probably cut a few spells for more Demons, and I might play one or two copies of Disciple of Griselbrand. As for the sideboard, I'm not too impressed by Illness in the Ranks and might rather just pay full price and get Curse of Death's Hold.