FrostMourne is a legendary sword from world of warcraft, worn by the LichKing (arthas). FrostMourne is not optainable in wow but it got a brother, a legendary battle axe named ShadowMorne,

code: /give @p minecraft:diamond_sword 1 0 {display:{Name:"FrostMourne",Lore:["Let them come... frostmourne hungers."]},ench:[{id:16,lvl:80},{id:19,lvl:80},{id:20,lvl:80},{id:34,lvl:80}],StoredEnchantments:[{id:16,lvl:80},{id:19,lvl:80},{id:34,lvl:80}],AttributeModifiers:[{AttributeName:"generic.maxHealth",Name:"generic.maxHealth",Amount:103,Operation:0,UUIDMost:15709,UUIDLeast:117451}]}

i have puted abit easter eggs from the game into the sword:

1. the sword is named FrostMourne

2. all enchants are lvl 80, wich was max lvl in the, wrath of the LichKing expansion

3. the sword says. Let them come... frostmourne hungers. as LichKing said in the patch 3.3: fall of the lich king -
Patch video.

4.the sword gives you +103 health, thats becuse LichKing (difficulty 25 hc) have 103 million health.