Premonition of the US Sen'jin server, Alliance side, becomes the first US guild and the third guild in the world to get the Alone in the Darkness Achievement, thereby also earning the Death's Demise title. Alone in the Darkness requires you to defeat Yogg Saron without the help of any Keepers, thereby eschewing the requisite buffs they give. They're hot on the heels of Paragon, which got the EU first kill, and Stars, which got the world first (and by extension, Taiwan first) kill.
This is definitely a difficult achievement, and, some argue, possibly even harder than killing Algalon himself. Congratulations, Premonition!