Whether it's a hidden corner of a StarCraft II campaign map or something mentioned in a WoW quest, Blizzard always finds the weirdest ways to hide easter eggs in their games. This time it's a Diablo III-themed one in Heroes of the Storm.
So to unlock this particular one, you need to be on The Battlefield of Eternity map. There's a sword, shown in friiserburn's video, hidden on the map and you have to click it repeatedly. The sword will finally drop with a green light, just like a Diablo III set item, but picking it up is not possible. It will result in a "Your inventory is full" message.
But after the error message, at the end of the game, a Treasure Goblin, also from Diablo III, appears once the core is destroyed.