So what is this sword exactly? Information is very scarce at the moment. No stats have been released yet but we do know that it is a two-hander axe, aimed at melee classes. This means primarily warriors, paladins, and death knights (both dps and tanks). While it could possibly be wielded by other classes, Ghostcrawler has iterated (source) that the stats will favor the above classes.
It will be available in the Icecrown Citadel, the same way that Val'anyr, Hammer of Ancient Kings was available from Ulduar. Myself, I wonder about the name of the weapon. It sustains a striking resemblance to the Lich King's signature weapon, Frostmourne. I hope there is some rational lore attached to the name, for otherwise it is an uncomfortable recycling of ideas. By all means, I am thrilled that there is a legendary melee weapon coming out, as the last one was clearly a caster weapon, but will feel like a slight cheat if it is just a makeshift version of the real thing.
Some people point out strongly that the legendary weapon from Icecrown Citadel should have been Frostmourne itself, but I strongly disagree, as do most who care at least a tiny bit about the game's lore aspects. To allow it to be fallen into player hands would mean that it would have to be defined into logical parameters would only demean and weaken it, reducing it to "just another legendary" since it obviously could not corrupt the player character the same way it was responsible for creating the most iconic villain in the game.
No, no, no I say. Frostmourne is unique because it is undefined and its powers a mystery to us. It belongs in the possession of the Lich King, where it maintains its power and provides us with an unforgettable gaming experience.