Horizon Zero Dawn, despite the silly name, is a great game. And for our purposes a fine fit, like Banner Saga before it, for OSR conversion.
This is going to be spoilered to all hell.
Primer for Players
Far future Earth. Green apocalypse. Civilization is pre-industrial and, as far as we know, is centered in the western US (Colorado, Montana, Utah, Arizona in particular). The primary remnants of the old world are the machine-creatures found throughout the wilderness. Beyond that it's just some rusting ruins, hidden bunkers, and the occasional bit of salvage (metals and ceramics, no plastics). The robots have been getting more aggressive over the last decade or so (the Derangement).
Primer for DMs
There was a grey-goo scenario in the 2060s. self-replicating, biomass-eating warbots built without a remote shutdown backdoor, all that good stuff. Total biosphere collapse, extinction of all life on Earth. Project Zero Dawn was a last-ditch reboot effort - build an AI (GAIA) capable of brute-forcing the shutdown the Faro warbots, and then clean up and re-seed the biosphere from seed and gene banks using it's own robots. It succeeded (hence, why there is a game), but the subfunction APOLLO was sabotaged, wiping out the entire recorded history of humanity and destroying any resources for the humans raised in the Cradle facilities (this is why there are no cultural remnants of the Old World to be found in any of the post-ZD civilizations) The first Cradles started releasing people into the wild some 700 years ago or so. Current year is 3040.
- Common weapons are simple and pretty standard: bows, spears, swords, axes, slings. Simple explosives and rapid-crossbows are less common but still widespread.More exotic weapons include tripwire launchers and restraining ropecasters.
- The Osteran have been able to develop proper cannons, and some Carja have fire-spear launchers of their own. Any other firearms are scavenged off of robots and usually in possession of the Eclipse.
- HZD does not match up to any pre-existing historical era particularly well, but it is safe enough to think "anachronistic bronze age".
- The largest animal seen in-game is a wild boar. Some livestock can be presumed, but it seems most of the bigger animals have gone extinct for good.
- Primary currency is metal shards chipped off of machine plating. Ruinations scrap works just fine to this end.
There are four primary cultures featured in the game, plus three minor.
- Nora - Matriarchal, isolationist. Strict taboos on personal behavior and even stricter against exploration of ruins or meddling with machines, punishable through exile. Keep to themselves in their Sacred Lands, which are in the general Colorado area.
- Carja - Theocratic cultural hub, largest civilization in the known world. Consider their king the vessel of the divine sun. The old king (Jiran) was a murderous tyrant and was overthrown by his son in a civil war. The current king (Avad) is a pretty cool dude: banned slavery, opened the borders, more rights for women and foreigners. The Carja Sundom includes a lot of territory in the Utah / Arizona region.
- Osteran - Industrious, mercantile, forthright. Smiths, miners, mercenaries, tinkerers. They come from the Claim, which should be Idaho and possibly some of Montana.
- Banuk - Survivalist, technospiritualist. Clans formed via trial rather than bloodline. Shamans deal with machines and commune with the "Blue Light". They live in Ban-Ur, which begins in Wyoming and proceeds further north, possibly to Canada.
- Utaru - Pastoral, pacifist. Live in the Plainsong (Kansas and Nebraska, likely), and are rarely seen in the Sundom or surrounding territories.
- Tenakth - Cannibalistic raiders from further south. Nasty motherfuckers.
- Carja-in-Shadow - Supporters of the previous Carja king, driven westward by the terms of the ceasefire. Mostly the old priesthood trying to claim legitimacy through the six year old Itamen (youngest son of Jiran by a different wife) and acting as a cover for the Eclipse cult (who believes that the HADES subfunction is the Buried Shadow of their myth come to rebalance the disorder caused by Avad).
Only two other Cradle facilities are confirmed: one in Xinjiang Province (China) and one under Mt. Namuli (Mozambique). Potential sites (via in-game graphics only) include Greenland, Nigeria, northern France and eastern Russia.
Land west of the Rockies is entirely fucked, according to in-game sources.
The Medicine Pouch
A successful survival check (takes 1 hour) will gather enough medicinal herbs to restore 1 HP. You can store up to your max HP in your medicine pouch.
Equipment & Mods
The heaviest armor available is the equivalent of chain, with the lightest being no more than common clothing. What is more important is an armor's quality, which determines how many modification weaves can be applied. There are three ranks of quality for armor (1/2/3 mods) and for modifications themselves. Modifications provide damage reduction (1/2/3) against one of the following types.
Weapons can also be modded to do additional damage in the same manner, doing 1-3 additional damage when using the appropriate ammo type.
Machines are easy: just take your favorite wildlife from the Monster Manual, make it a robot, and give it a simple title related to what it does or what it's like ("Watcher", "Snapmaw", etc). While more exotic-appearing machines are certainly possible, it seems that HEPHAESTUS has kept it all within the boundaries of creatures that once existed on Earth (GAIA was quite fond of megafauna, it turned out.)
Major machine classifications include: terraformer (maintain soil fertility), purifier (clean & maintain air and water), acquisition (gathering and recycling resources), transport (moving resources), recon (detection, guiding other machines to needed locations), communication (maintaining network), combat (post-Derangement hunter-killers) and chariot (Old World warbots).
Each creature has a chance of dropping a lens (40%) or a heart (10%). These are worth, respectively, 10xHD and 50xHD shards.
Machines are generated in HEPHASTUS-run cauldron facilities.