Obviously the idea of making hard mode encounters was welcomed nicely. This is how Blizzard introduced their "final release" of a hard mode with the Sartharion - Three drakes fight in Obsidian Sanctum.
When the Ulduar patch was released, Blizzard extended the hard modes even further. There fourteen bosses having hard modes in this instance: Flame Leviathan, XT-002, Assembly of Iron, Freya, Thorim, Hodir, Mimiron, Vezax, Yogg-Saron and Agalon the Observer. Speaking of Agalon, it's not a real hard mode because this fight actually has no normal mode.