That is arthas with the sword frostmourne. Its from Warcraft 3, and got incorporated into world of warcraft. And ovbiously i did think one ring before frostmourne, but frostmource fits just as well as the one ring.
Spoilers for WC3 below! beware!!
Arthas is the heir to human kingdom of goodness and is paladin in training. The Lich King decides to use him as his ultimate champion, so he sets him up. He forces arthas to kill innocent peasents to keep a zombie plague from spreading, and other such things. After a series of evil deeds for good ends, Arthas goes forth to find frostmourne, A blade powerful enough to defeat even the lichking. Arthas finds it, takes it.
Only Frostmourne is cursed and starts whispering in his ear and eating at his soul. Arthas returns home and greets his father the king, as usual, and then kills him.
Basically, a combo of the Lich Kings plans and frostmourne turned arthas from a hot-headed paladin and heir to to a kingdom into a hot-headed death knight out to kill all who oppose the lich king.
AKA Nazguls for those prefering the One Ring idea.