For a while now, "Corrupted" quality items have been showing up on the Steam Marketplace for Dota 2 (
http://steamcommunity.com/market/search?q=appid:570+corrupted[1] ). Their creation was part of a recent winter event in the game called Wraith Night. They come with a socketed gem called a "Foulfell Shard" (
http://dota2.gamepedia.com/Foulfell_Shard[2] ) which has a stat on it tracking Demons Imprisoned. Some people are saying that killing demonic characters like Doom, Shadow Fiend, Shadow Demon, Lion, Queen of Pain, etc. is causing the gem to count demons imprisoned. The sigil on the gem matches the one on the ranting madness post on the Wraith Night message board (
http://www.dota2.com/frostivus2013/day2/[3] ), which many people thought looked like the markings on Anti-Mage's face (
http://dota2.gamepedia.com/Anti-Mage[4] ). Being a demon hunter and being similar to Anti-Mage are two features of Anti-Mage's brother, the thus-far unreleased Dota hero Terrorblade (
http://dota2.gamepedia.com/Terrorblade[5] ).
I theorize that there is an engine of some sort at Valve counting the number of hits on these Foulfell Shards for demons killed in-game, and when it reaches a certain number, Terrorblade will be released. To facilitate this, more people should be playing demonic characters - and be sure to have a Corrupted item with a Foulfell Shard equipped, so that it will count when you kill other demonic characters.
Complicated, arcane methods of unlocking heroes are no stranger to the Dota franchise. In the original Warcraft mod, many obscure actions had to be performed to unlock new heroes. I think this is happening again.