<With the release of Shadowkeep slowly crawling its way to the horizon, I think some speculation is in order. This will be about the enemies that we'll be encountering and their overall feel and even a possible hint at their motives.
[b]The Taken?[/b]: While no Taken are shown on the Moon, the Taken forces that infiltrated the Ascendant Leviathan while Gahlran led the Hive in reality were led by an Ogre and Centurion. Both of them were "Born of the Hellmouth". This suggests something on the Moon is capable of Taking or otherwise producing Taken.
[b]The Red Legion[/b]: The broken Cabal Red Legion makes a small appearance on the Moon, specifically in the First Light area of the map. They are most likely remnants of the missions there during the Red War. They are the most minor enemy faction on the Moon.
[b]The Fallen[/b]: The Fallen House of Dusk will be returning to the Moon once more, perhaps implying that the House of Exile joined the group. They will be taking residence in the seemingly crashed ship that once hovered above the Temple of Crota, the Wolfship Kaliks-syn. Their presence isn't too notable, but they're there to add variety.
[b]Unnamed Hive Brood[/b]: These are the Hive we've been fighting since our trip to Titan. They seem to be minor and only in areas once occupied by the other broods, but they're there nonetheless. They do not seem to be at conflict with the other Hive brood present on the Moon and are most likely there for assistance.
[b]The Hidden Swarm[/b]: [url=https://pbs.twimg.com/media/D_iT3znXYAMtw_S.jpg:large]These[/url] guys are the main Hive force on the Moon. Veterans may recognize their red color and apparent love for sticking chains into their bodies. Their armor appears to be a mix of the D1 and D2 art styles. They took residence in and possibly built the massive scarlet fortress that lays around the Hellmouth. The fortress digs deep into the Moon's crust, either cementing the massive caves the Hellmouth dug, or they made new holes for breeding or excavation purposes. They may be linked to Savathun, but it's unknown whether the Moon is a side project or something else.
[b]The Nightmares/Phantasms[/b]: [url=https://pbs.twimg.com/media/D_iT1epX4AAVe1g.jpg:large]These[/url] monstrous entities are the recreations of the foes slain by the Young Wolf over the years. Their exact origin is unknown, but somehow Eris managed to release them onto the Moon's surface. When they idle, they appear blood red and slightly transparent, with massive red veins protruding from their bodies. When approached, they solidify into their form and act accordingly. Bungie said these things were resurrected by the Darkness, their shadowy look and the veins that appear when idle back this up.
One of the most interesting thing about the Nightmares is that they don't seem to care where their allegiance lies, as long as it isn't with us. Nightmare Taniks and Skolas got together with the Fallen group in the crashed Ketch and they didn't try to kill each other. Not only that, but the Cabal band on the Moon allied with Nightmare Ghaul and the Vex allied with Nightmare Zydron. All of these events so far have shown cooperation rather than destruction, as if they went with what they were doing before this all happened.
Whatever Eris accidentally did and whatever the Hive seem to be intentionally doing added a psychological element to the Nightmares. Physical manifestations of enemies that we hated or had difficulties taking down. However, there are also manifestations of the souls of Guardians, seen surrounding Nightmare Crota and hovering around the scarlet citadel on the surface. They don't attack, but they don't act natural, either.
Nightmare Omnigul is fought in the [Chamber of Night?]
Nightmare Phogoth is fought in the Summoning Pits
Nightmare Ghaul is fought in the First Light map
Nightmare Taniks is fought in the [Seeder Launch area?]
Nightmare Skolas is fought in the Wolfship Kaliks-syn
Nightmare Crota is fought twice, once in the same room you fought his Soul and once in a recreation of his Throne World where he died last time
Nightmare Zydron is fought beneath the Moon's surface, before a suspicious Vex Gate
[b]The Sol Divisive[/b]: As seen at the bottom of picture 2, the Vex are backing up Nightmare Zydron. These aren't the programming we've been fighting throughout D2 though. These are Vex from the Black Garden, the Sol Divisive. They were the architects of the entire Solar System campaign in D1, and they were the primary worshipers of the Heart of the Black Garden. The Vex gate they're guarding with Zydron is likely another gate to the Black Garden, as the Mars one was destroyed and the Shadowkeep raid is in the Black Garden.
Their purpose now is unknown. After the Heart was destroyed, the Divisive brought in the Undying Mind, an old Precursor Hydra, to detach the Garden from reality and back into the void it came from. It failed, but now it seems that the Vex got their act together, as the Garden lays suspiciously within view of the Earth and Moon.
[b]Something Beneath the Surface[/b]: This isn't just about the Hive, or the Vex, or the Taken. Something undefined is beneath the Moon's surface, and it's not the Hive. Something different rests beneath the Moon, perhaps a vestige or a worm on a hook. Something is watching, and I have absolutely no clue how or when it got there.>