Alongside the release of Season of the Haunted, Destiny 2 also received a new dungeon, Duality, that sees you enter the mind of Calus, disgraced emperor of the Cabal and fighting the nightmares of his past. If you think your guardian is up for the task, talk to Suraya Hawthorne in the Tower Bazaar and accept the quest "A Single Thread", which will give your guardian access to the dungeon Duality and all of its highly sought rewards.
Your fire team will need to start their journey on the moon, aboard the derelict Leviathan. Your fire team will be immediately entering Calus’s mindscape, a place awash in blue light, its architecture a mix of Cabal, Leviathan, and Calus’s own brand of ostentatiousness. Most pressingly however, there’s a gate blocking your path, and beyond that, a bell that introduces you and your fireteam to the main mechanic of this dungeon.
Calus’s mind has not fared well, and while there is some relative safety in the calming blue mindscape, there is also a land of madness and nightmare that plagues him, living side by side with his mindscape. The bells throughout the dungeon act as gateways between the blue mindscape and the red nightmare land. In the mindscape, your guardian or your fire team can shoot at the bell from anywhere on the map and be transported to the nightmare, but this is not true of the return trip. In the nightmare, your entire fireteam will need to be in proximity of the bell being rung, as anyone who finds themselves too far away from the bell will die.
The mindscape and the nightmare have similar maps, but they’re not identical. Shooting the bell in this first section will show you that much, as the gate blocking your path doesn’t exist in the nightmare realm, allowing your team to walk up to the bell and return to the mindscape. The first leg of the dungeon is a slow burn, letting your team get used to the bell mechanic with some platforming with a small side of combat as you and your team make their way to the first encounter of the dungeon. Nightmare of Gahlran, Sorrow Bearer.
As might be suggested by their name, Nightmare of Gahlran lives in the red nightmare world. But don’t just go in guns blazing, the Nightmare of Gahlran completely immune to all damage until certain conditions are met. Not only that, but from this encounter onwards, there are some restrictions to your bell usage. Every time your team enters the nightmare world, two Bell keepers will spawn near the bell. Until they are killed, the bell cannot be used again. In addition, the nightmare has grown volatile due to your intrusion, so you are now operating under a time constraint while in the Nightmare, and it can cause a party wipe if you aren’t careful.
In the mindscape, there are 4 side rooms, each bearing one of 4 symbols. In the nightmare, there are 2 essences, each marked with one of the same 4 symbols. Each will need to be picked up by a guardian. Upon getting back to the mindscape, the 2 guardians with Essence will need to find the side room with the appropriate symbol and enter it. The third member of the fireteam will then need to activate the bell. The two guardians in the side rooms will then be attacked by several Visions of Gahlran that they will need to defeat, as the Nightmare of Gahlran will only be vulnerable to damage once all of them have been killed. If your team has to leave the Nightmare, you will have to repeat this process to make him vulnerable once again.
Another set of platforming sections follows, along with a small puzzle where you must point statues towards the center of the room in the mindscape, while the controls can only be accessed in the nightmare. Doing so correctly will open up a hole in the floor, and allow your team to access the vault.
The following encounter follows a similar flow to the first. A central room in the mindscape shows 2 symbols, and your team must find the corresponding essences in the nightmare and return them to the mindscape. Each essence is being held in the area corresponding to the essence’s symbol. It is being held by a Cabal Centurion who will only appear after you clear the area of Cabal. After returning both essences back to the mindscape, you’ll attract the attention of the first of the three bosses of this encounter. Nightmare of Ka’hok, Sworn of Umun’arath. Your team will need to repeat this same process to fight the remaining two bosses, Nightmare of Emonut, Sworn of Moli Imoli and Nightmare of Uroa, Sworn of Shayotet.
A final platforming section is now all that separates your fire team from the final boss of the dungeon, Nightmare of Caiatl, Princess-Imperial. She is, of course, immune to damage to begin with, and your team will find her in front of a new type of bell, a much larger one known as the Bell of Conquest. The basics of symbol finding remain the same. Two will appear in the mindscape. Go to the corresponding area in the nightmare and kill the Scion that holds the corresponding essence to claim it for yourself. Securing a total of 4 essences will allow your team to ring the bell of Conquest, signaling the start of the next phase.
Your team will be teleported into Nightmare, and Caiatl will run to one of the other bells in the room. Your team needs to get to that bell first, wait until she is close, and then ring the bell. Your team won’t be teleported back to the mindscape this time, instead, Caiatl will be stunned, and assuming your team stood close enough to the bell, you’ll all be given a temporary but sorely needed damage buff, as Caiatl has an unfathomably large health bar without it. Repeat these steps as needed to finally slay the nightmare, and claim your just rewards.
And what rewards they are. As shown in this image by Nemuthegreat, Duality has a large pool of high quality gear, with no weekly lockout for any of the encounters mentioned. And it’s little wonder why, with heavily armed Cabal lurking around every corner in Calus’s mind, Duality has been called one of the most difficult combat challenges in a Destiny 2 dungeon. With so much to risk, and so much reward, you’ll want the best of the best to be on your fireteam. Good luck guardian, lest you succumb to the nightmares.