Ulduar is just one of the many World of Warcraft raids that this week's leak points to as being the next Hearthstone adventure. But for those of us who haven't played WoW, what exactly is Ulduar? What's the backstory? What kind of minions and bosses are we looking at? Introduced in the Wrath of the Lich King expansion and widely considered one of the best raids ever released, Ulduar is full of wonderful content and lore ripe for conversion to Hearthstone.
"Since discovering Ulduar in the Storm Peaks, adventurers have journeyed to the titan city to learn more about its mysterious past. Further exploration has revealed that the magnificent complex is above all a prison, intended to permanently confine the Old God of death, Yogg-Saron. This ancient horror has corrupted its guards, and now it is breaking through the last of its restraints.

Faced with the peril of Yogg-Saron's imminent freedom, a band of mortals has made preparations for a sweeping assault on the city. Overcoming the legions of iron minions and towering defenses at the city's entrance will take tremendous strength, but even that great victory may not be enough to overcome the madness that dwells below."

via Battle.net

"Ulduar is a raid dungeon in the Titan complex of Ulduar, located within the The Storm Peaks," writes Wowhead Site Director Alexis Kusy. "It serves as the prison of the old god Yogg-Saron as well as the current residence of most of the titanic watchers who have fallen under his influence. It is notable for its interactive ways of starting hardmodes--instead of switching the difficulty of the encounter before fighting, players would have to start the encounter in a certain way to unlock the hardmode. For example: defeating Thorim's gauntlet in 3 minutes or less would trigger his hardmode."
She continues, "The Yogg-Saron hardmode is especially notable for having varying degrees of difficulty. In the basic version of the encounter, players receive assistance in the form of buffs from all four Keepers. If players do not speak to a Keeper before pulling Yogg, they will lose that Keeper's buff."

This gives the developers at Blizzard an chance to create a really interesting way of unlocking the Heroic version of each boss. What if you instead had to meet certain goals before you could unlock them rather than just doing so automatically after they're defeated? The community seems to really appreciate when Hearthstone goes above and beyond when integrating elements from World of Warcraft itself and this is a great opportunity to do so.

We've already seen many notable characters be added to Hearthstone that originate from here including Flame Leviathan, a boss from The Siege of Ulduar, Mimiron (or at least Mimiron's Head) and V-07-TR-0N from The Keepers of Ulduar, and Yogg-Saron, Hope's End from The Descent into Madness. You also encounter one or two minor minions such as Magma Rager and Kirin Tor Mage, but also other major characters like Alexstrasza, Malygos, , Ysera, and Brann Bronzebeard.

Especially considering the reception of Whispers of the Old Gods, more specifically Yogg-Saron and how the community has really rallied behind it - easily one of the most popular minions and sets to ever exist - it would make a lot of sense to so quickly turn around and capitalize on this. And for every one of those minions Blizzard would be able to flesh them out some more and provide more means for players to connect and understand them, especially those who aren't all that familiar with Warcraft itself.

And lets be honest, people have been clamoring for Algalon the Observer since the Naxxramas first made its debut.

Our sister site Wowhead has more information if you're interested.
Algalon card source: Reddit