Ragnarok

Everybody wants Ragnarok to bring the destruction of Asgard as we know it. This means that Earth will be all alone (well, not exactly, since the Guardians of the Galaxy will arrive) when the Mad Titan arrives.

But let's not get ahead of ourselves. Everybody knows that Hela is the movie’s main antagonist, even though other villains will also appear (for example, Karl Urban's Skurge). And Hela is a powerful Asgardian, the Goddess of Death, ruler of Hel. And Hel is populated with the spirits of the dead.

Basically, Hela commands the legions who have died, except for the heroic Asgardians and humans. So it isn't hard to believe that she possesses a powerful artifact with which she can control all of these spirits. In other words, the rumor says that the last Infinity Stone, the Soul Stone, will appear in Thor: Ragnarok. And it makes sense, even if Hela is the MCU's Death.