Shortly after Destiny 2 was announced, director Luke Smith said something that interested me greatly. It was something to the effect of "Destiny 2 isn't a sequel. Destiny 1 is a prequel."
That's a mentality that wasn't wholly apparent in the campaign. To me, the story of Ghaul, the Red Legion and the retaking of The Last City was perfectly self-contained. It was an episode. But the content didn't end there. Strikes showed new threats: The Taken, Savathun, the Vex. The Raid, further, showed us Emperor Calus, and his bizarre trial. On the subject of Trials, the Nine, offered their own, as a replacement for those of Osiris.
Was this all just a test? The Nine saw our war with the Red Legion as a test. They offer us further tests in the Trials of the Nine. Calus, in particular, has an entire series of tests onboard his Leviathan. Beat the final boss, his animatronic replica, and you'll enter a treasure room---surrounded by even more of his hulking, robotic forms.
In fact, the campaign itself revolves around these themes. Ghaul repeatedly rejects the Consul's urging to seize The Traveler's Light. He seeks to be chosen. In his discourse with The Speaker, his repeatedly brags about the Trials he's faced. He overcomes his origin as a runt and in his own words, he's slain gods. It's not hard to imagine that he, at some point, discovered a god of Oryx's power and defeated it, far more easily than we did.
It's on these terms that Ghaul appeals to The Traveler. It's on these terms, the Nine assess our worth, and it's on these terms that Calus finds us worthy and spares us from being devoured by The Leviathan.
Make a notable kill in Trials and the Emissary is awed, breathlessly muttering, "Are these…the ones?"
Even more interestingly, upon defeating mecha-Calus, the dying machine plies the player: "Do you still believe you're on the right side? Mull it over and enjoy my gifts to you. I possess the means to true agency beyond you feeble Light. Seek me out and perhaps I'll show you how to grow fat from strength."
We know this to be a test that he's applied to a number of races with only a few passing and surviving. Likewise, the Nine appear to have multiple races making up "Agents of the Nine." The first Agent of the Nine, Xur, is Jovian; we know his entire race to have been former humans, transformed and made servants of The Nine. With Destiny 2, we're introduced to The Emissary, also an Agent of the Nine, yet an entirely separate race.
Like Xur, her will does not seem to be her own. She telekinetically communicates with someone other than ourselves, and her mind seems adrift. Like Xur, she's often mumbling to herself. I believe the races made "agents" of the Nine have been telepathically assimilated. That's a theory though. All we do know is that The Nine possess multiple races.
In the same way, we know that Emperor Calus has traveled to the deepest reaches of the universe and collected races such as the Clipse and Sindu. These races join Calus' Loyalist force, killing high-ranking members of Ghaul's Red Legion.
Both these forces are keenly interested in humanity. They create Trials for them to complete, presumably to recruit or test them.
So the key theme of Destiny 2 is "Testing"
This theme isn't merely an abstraction of the overall game. Remember how this article started? Luke Smith claimed that Destiny 1 is a prequel to Destiny 2. The theme of testing is preceded by the central theme of Destiny 1.
Don't remember it? I don't blame you. It's not readily apparent. But it is consistently visible. That theme is Causality.
In my article on The Taken King's Paradox mission, I argued that Destiny is a game about causality. The Vex operate in near-infinite timelines which they fight to reconcile with their goals of timeless domination. But for all their failures, they see humanity as the only source of causality that makes our timeline exceptional.
It's for this reason that the events of Destiny are notable. In a world of gods, monsters and magic, only the introduction of the player can bend the universe away from its natural conclusion. So when The Vex are trapped in a universe of inevitable defeat by The Taken, they trust only The Guardian to save them.
So let's contrast The Vex and The Hive with The Nine and The Loyalists. The Vex and Hive are seeking for solutions to The Darkness. The Vex determine that worshipping The Darkness is the logical path to survival; the Hive determine that galactic supremacy is the ritualistic path to survival. The Vex burn away timeline of their inferior, less successful iterations in order to find eternal life. The Hive slaughter thousands of races across the universe in order to declare themselves worthy of The Darkness.
So if Destiny 1 is a prequel, or an introduction, to Destiny, Luke Smith is implying that the central conflict of Destiny 2 is the primary one of the series as a whole. This relegates the preceding game to a different role: one that sets the stage and introduces the pieces.
Where am I going with this? I've meandered a bit, but--like Destiny 1--in order to put the appropriate pieces on the board.
Destiny 1 is about fighting races that are responding to The Darkness. Our path to victory is through our empowerment via causality. We establish ourselves as a determinant force within the universe.
Destiny 2 kicks off the real story of Destiny. Having demonstrated our exceptionalism, new forces enter from the fringes of our known universe to see us for themselves. All to a certain end. It's plausible, likely even, that Calus and The Nine have similar goals.
This goal? There's no way of saying. My easiest assumption is that they, too, seek to fight The Darkness. But the demotion of Destiny 1 to a "prequel" makes me doubt this. Whatever the universe's central conflict is, I believe it's something Bungie has yet to reveal. But it certainly exists. And Bungie's building it to be something very, very formidable.
(So no, I don't believe the end-credit scene is 'The Darkness.')