World-first Heroic: Alone in the Darkness? "Didn't Exodus already get that?" you may ask. Well, they did get the achievement, but they used an exploit, and were slapped on the wrist to the tune of a 72 hour suspension. Which means: the stage was open for a legitimate world-first!
In a break from the tradition of European guilds getting major world-first accomplishments, Chinese guild Stars (playing on the Taiwanese servers) has apparently completed the achievement legitimately. They got that nifty Mimiron's Head mount, too.
In case you're scratching your head wondering what this is all about, during the Yogg-Saron fight (the last boss of Ulduar), the four Keepers of Ulduar (Freya, Thorim, Hodir, and Mimiron) help you out. You can choose how many of the Keepers you want to activate for the fight, and if you use fewer than all of them, it's hard-mode. "Alone in the Darkness" is Yogg-Saron with zero keepers: as hard as it gets.
Congratulations to Stars on a truly impressive feat.