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The very first time a player kills the Lich King after completing their  Shadowmourne, an item called the  Unsealed Chest will drop. It is only visible and lootable by the wielder of the Shadowmourne. Inside it are five quest items. Although only the recipient of the chest can turn in these quests, the items rewarded by completing them are Bind on Equip/Use and can be shared with raidmates who helped you complete your legendary or sold on the auction house.

This quest rewards the  Reins of the Crimson Deathcharger, a mount similar to the Death Knight class mount.