After you come back from looking at the river of fire, Thanos is stomping toward you, and he is NOT in a good mood. Pause and save the game; this is an extremely time-limited puzzle.
Hint 2
Man, Thanos is an animal! Wouldn't you love to put him in a cage?
Hint 3
Sounds like a good idea to me. Did you notice that he's standing in the area where you were just caged?
Hint 4
This is where you need to use that "activation code" you saw in the note from Xenia's box.