I have seen my fair share of difficult bosses in World of Warcraft, but Blackhand looks to be one of the toughest.
To give you an idea of the abilities to conquer, just read through WoW Head's Strategy  :
- Slab bombs that explode after 3 seconds and are spread throughout the fight
- Falling debris that occurs throughout the map
- Molten slag fills the arena throughout the first part of the fight, limiting movement area
- Siegemakers spawning throughout Phase II that spread "blaze" ground fire and target players
- Phase III spreads fire damage throughout the raid, constantly, until Blackhand is killed
- Shattering Smash hits your tanks and must be shared throughout raid at later stages, wiping threat, and putting a stun on you; becoming Massive Shattering Smash in Phase III
A handful of guilds have conquered it since its release 27 days ago, here's Method (EU) with the world's first Blackhand kill after 5 days .
I may not have reached many end-game bosses, but WoW bosses are no pushovers, and just watching the chaos I can feel how many wipes it would take to make it just past the first phase alone. Impressive stuff from the guilds that have managed to conquer this latest challenge.
World of Warcraft is still going strong with a large subscriber base and their latest Expansion, Warlords of Draenor, was released this past November with positive reviews all around.
 WoWHead- Blackhand Strategy Guide - http://www.wowhead.com/guides/raiding/blackrock-foundry/blackhand-strategy-guide
 Kotaku - Watch the World's First Kill Of One of the Hardest Bosses - http://kotaku.com/watch-the-world-first-kill-of-one-of-the-hardest-bosses-1690259151