The Soul Stone's Sacrifice is a Killer
Some might take issue with the level of security reserved for only this one of the Infinity Stones, since the magic explained by Red Skull suggests the Stone exists... outside of the normal universe, in one sense or another. The Soul Stone requires a test to both prove the worthiness of the person coming to claim it - and its power over the souls of all living things - and to demand an exchange. That final detail is, once again, a unique feature of this Stone alone.
To claim the Stone of Souls, a single soul must be sacrificed. Not that of the prospective master (this isn't a story of devils or bargaining), but of someone that person truly loves. From there, the process seems simple enough: shove that person off the edge of the cliff, wait until they reach the bottom and offer up their soul (lose it? Send it into the Stone? It's unclear) and the Souls Stone is yours.
Now, the wording of that bargain - that you are effectively trading one soul for another - raises some questions about the meaning of the loved one's death immediately. The final scenes of the movie may shed some light on this, but that's for future movies to reveal.
Once Thanos drops his loved one to their death, a bright light erupts, the storm overhead parts, and a sudden rush of cosmic energy forces even Thanos to lower his head and close his eyes. In an instant he awakens far below the heights of the mountain in a shallow pool of water. Sacrifice made, Soul Stone in hand.
Why The Soul Stone Was The Last To Be Found
Some fans will certainly be disappointed or underwhelmed to find that the Soul Stone wasn't really hidden anywhere in the known Marvel Universe. Not hiding in Heimdall's chest, not hiding in Odin's eye socket, not hiding in the heart of Wakanda's vibranium meteor. But fans didn't know before now that the final Infinity Stone would make demands of Thanos, composing a major part of his character in the movie. Not just what he was willing to do to see his plan through... but the kind of villain that he is NOT.
Presumably, whoever left the Soul Stone in this location chose the requirements of its passing to a new master carefully. Fans can debate just what is proven by someone willing to murder a loved one for power over the dead (if anything) but it seems the more important part of the challenge is the same one Gamora first misjudges.
The Soul Stone will never be offered to someone who has not loved, or does not have the capacity or willingness to love. Which explains why Red Skull was forced only to guard it. But by revealing that he truly does love Gamora - a fact doubly proven when killing her works - the retrieval of the Soul Stone cements Thanos as the only kind of character who can sell his master plan to the audience.
The desire to wipe out half of the universe may seem purely evil. Paying such a price so that the universe may blossom anew (once the grief wears off) wouldn't be worth it for most, or any hero. But regardless of whether you think Thanos a madman or a prophet, the Soul Stone twist proves that he has belief.
He loves, has loved, and is willing to sacrifice that to better the universe instead. Compare that to the satisfaction of Odin plucking the Stone from his eye socket, or Heimdall popping it out of his chest plate, and it's hard to say the filmmakers didn't make the right call.
So much for the T-H-A-N-O-S theory.
- Avengers: Infinity War / The Avengers 3 (2018) release date: Apr 27, 2018