Destiny 2's Season of the Haunted is working through a compelling, character-driven story, one that dives into the personal traumas of some of the game's major characters as they try to deal with the new threat presented by Calus and the Derelict Leviathan. Like the best of Destiny 2's recent stories, it relies heavily on player knowledge from past seasons and expansions.
Generally, this is great, because with Bungie building on events of the recent past, Destiny 2 feels like a world that is changing as a result of the things that happen within it. But if you missed certain previous parts of the story, like the Forsaken or Shadowkeep expansions, this season might have you a little lost.
There's a lot of backstory you need to know to fully appreciate Season of the Haunted, stretching all the way back to vanilla Destiny 2, the Leviathan raid, and more. Here's a quick history lesson that will get you up to speed on everything you need to know about the Leviathan, Nightmares, Caiatl, Crow, and everything else this season--regardless of whether you've been around since the Destiny alpha, or just jumped in with The Witch Queen.
A quick history of the Cabal
The conquest-driven Cabal were briefly steered away from war by Calus after his revolution, but his lack of military leadership led to his undoing.
Season of the Haunted continues the story of the Cabal empire, which has held permanent residence in Earth's solar system since back in the days of the original Destiny. They've changed drastically since then, however, with several different factions operating in the solar system at the same time.
The Cabal as we know them are a warlike group of conquerors, and that's rooted in their deep traditions. They value things like honor and trials by combat; they're sort of like Star Trek's Klingons, and you can see in their architecture and unit names that Bungie draws a lot from the Roman Empire to inform the Cabal. The group is actually made up of a couple of species operating under the banner of the Cabal homeworld, Torobatl, which was the seat of an empire that stretched across a vast number of worlds.
At some point in the past, a new emperor named Calus gained power as part of a popular revolution against the military that controlled the government. A narcissist and hedonist, Calus wasn't interested in conquering anything and mostly was all about his own personal pleasure. He reformed the Cabal empire, taking power from the military Praetoriate that had formerly ruled, and instead offering the people feasts and gladiator bloodsports. A lot of the traditional Cabal chafed at Calus's decisions, and from what we learn in the Duality dungeon, it sounds like Calus had some or all of those Praetoriate dissenters purged.
Calus mixed ruthlessness with excess and opulence, and at least to hear him tell it, his people were happy with his rule. During his time as emperor, Calus had a daughter named Caiatl, who he loved dearly. But Calus was also self-centered and jealous; he would be kind to Caiatl one moment, and cruel to her the next. In the recent lore of Season of the Haunted, we learned about how Calus gave Caiatl a war beast for her birthday as a child, which she came to love intensely. War beasts hold a place of honor in Cabal society, and Caiatl grew extremely close to hers, only for Calus to grow jealous that she loved the pet more than she loved him. Calus had the war beast butchered, and that seems to have been the moment that solidified Caiatl's hate for her father.
Ghaul endeared himself to Calus, but the emperor had no idea his favorite Primus was plotting a coup.
During this same period, Calus took notice of a particularly fierce gladiator named Ghaul, an albino Cabal who'd been left to die as a baby because he was a runt. As a gladiator, Ghaul proved himself in battle over and over, and Calus was so impressed that he pulled Ghaul from the gladiator life and elevated him to the leader of his fiercest military force, the Red Legion. Ghaul became known as the "Ghost Primus" and Calus considered him something of a son. What Calus didn't know was that after Ghaul had been abandoned as an infant, he was found and raised by The Consul, a former member of the Praetoriate. Together, the Consul and Ghaul enacted a lengthy plan to get revenge against the emperor.
Meanwhile, as Calus threw parties, the Cabal empire came under threat, with Hive forces attacking its outlying worlds. Calus didn't think that was such a big deal, but some of his generals and advisers, and even his own daughter, saw his failure to act as a failure to lead. They definitely weren't wrong, either--the Hive were massively powerful and dangerous, conquering and destroying worlds as the Cabal forces tried and failed to hold them back, without any larger plan in place. When Ghaul was ready to enact his plan to overthrow Calus, he was able to get the backing of key military personnel and advisers, as well as the Princess-Imperial herself, because of this threat. Caiatl was the lynchpin of the plan.
Ghaul launched his plot in the middle of the night; what would later be known as the Midnight Coup. The Ghost Primus stormed the palace, and Calus hurried to his throne room to retrieve some special object he thought would save him from the traitors. It's not exactly clear this object was, but lore implies it might have been an Ahamkara bone. When Calus arrived in the throne room, however, Caiatl was already waiting for him. She destroyed the object, sealing Calus's fate.
We learn in the Duality dungeon that Ghaul was ready to execute Calus as he took power, naming himself to a new position: Dominus. However, apparently, Caiatl convinced him otherwise. Instead, Calus was exiled from the Cabal empire. He and his loyalists were placed on a huge ship called the Leviathan, which was sent out to drift through the dark reaches of space forever, its navigation systems destroyed. Aboard the Leviathan, Calus would eventually come into contact with the Darkness, and through it, Destiny 2's overarching villain, the Witness. After that, he would wind up in Earth's solar system.
Dominus Ghaul and the Red War
With the Red War, Ghaul attempted to steal the Traveler's Light for himself. He was ultimately defeated, but not before fundamentally altering the course of events in Earth's solar system.
After taking power, Ghaul was in contact with Cabal forces in the Sol system, where they had dug in on Mars sometime between the Collapse and the present day. It would ultimately be a losing battle; when the Hive god Oryx showed up during Destiny's The Taken King expansion, he used his power to circumvent the free will of other species against the Cabal, "taking" them and forcing them into his army. In response, the Cabal attacked Oryx's ship, the Dreadnaught, and tried to destroy it. They were stopped by the Guardians, but not before they sent a distress signal back to the empire. Ghaul received that signal and started to enact a plan, even as the Guardians defeated Oryx and drove the Cabal back.
Destiny 2's vanilla campaign was all about Ghaul. After receiving the distress call, the Dominus departed with the Red Legion for Earth's solar system. When he arrived, he caught the Guardians unawares, launching a sneak attack on the Last City. Ghaul executed a plan to try to take the power of the Traveler for himself: Using special technology he created a cage around the Traveler, cutting the Guardians off from their Light. Without their powers or their immortality, the Guardians struggled to fight the Red Legion and were scattered throughout the solar system. Destiny 2's story was all about finding a way to regain the Light and, eventually, defeat Ghaul--with the help of the Traveler, which had been dormant for centuries. Waking the Traveler set all the events we've seen for years in Destiny 2 into motion.