I've been going through the cards and can't find any hints as to what is inside the traveler. Was it a ship of a really advanced race like the forerunners from halo? Or does it store the best specimens from the races it has tried to protect so far? Any other theories someone might have???
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The darkness. Which is why the hive and vex seek it. To release the Darkness from the prison called "The traveler". The prisons sole purpose is to find civilizations they deem strong enough to defend it and help them do so.