The Twilight Forest, in GTNH, is the first mod dimension you’ll visit in order to advance your tech progression (initially visiting to secure a source of Nickel to produce Invar, for your first EBF), as well as the starting point for all subsequent magic progression. The general progression of the Twilight Forest, for the very rare individual not already familiar with it (or familiar with later version mechanics that are not present in this version), can be quickly summarized as a dungeon/boss rush in a dimension borrowing from old fairy tales. Though less a direct, and more a broad inspiration from them.
In the 1.7 version of Twilight Forest the boss progression is entirely linear, and several biomes that house later bosses are “locked” until you’ve defeated the proceeding bosses. Should you enter such a biome anyway you’ll receive some manner of debuff, encouraging you to explore elsewhere, and a specific Kobold enemy holding a written book dropped on the Kobold’s death will spawn nearby. The written book will, rather plainly, spell out which boss you need to defeat in order to enter the biome you are not intended to enter yet. As a final note boss progression in the Twilight is based on linear Achievements, so breaking sequence is not possible. This system can be buggy, so it is advised to kill Twilight bosses with a Melee weapon so the killing achievement triggers reliably, and to pick up boss drops without a magnet so item collection achievements likewise trigger reliably. Yes, there are both boss killing quests and boss drop pickup achievements separately, and due to the linear nature of achievements you will need to complete both before moving on to the next boss. Keep that in mind in case you ever find yourself stuck in progression
The boss progression is Naga -> Twilight Lich -> Minoshroom -> Hydra -> Knight Phantom -> Ur-Ghast -> Yeti -> Snow Queen -> Giant Miner (technically). The Snow Queen is the final real boss mob, but there is one final biome unlocked by reaching and defeating the Giant Miner. (Specifically, after defeating the Snow Queen you can access the Troll Caves. Inside the Troll Caves are Magic Beans, which you plant to reach the Giant’s abode. Take the Giant Pickaxe from the Giant Miner to gain the Lamp of Cinders hidden in the Troll Cave, and use it to reach the Castle)
Tips and tricks: Normally in 1.7 versions a TiC Rapier is able to completely break the Twilight Lich fight, but in GTNH this interaction is patched as of version 2.4.0. That’s not to say TiC swords (or other weapons) are useless in that - or other - fights, though. Life Steal through Necrotic Bones can be very useful Unlike in later versions of Twilight Forest in GTNH you only need to pick up the Meef Stroganoff to open up the fire swamp. Eating it as a requirement was only added to later versions of Twilight Forest (though you should still eat it anyway to work towards more bonus hearts) The Scepter of Twilight shoots the same projectiles that you need to bounce back to break a Lich’s shields during the first phase of the fight. If you get a Scepter of Twilight from a Lich fight you can use it to basically skip the first phase of later Lich fights
To the cunning adventurer, the Twilight Forest in GTNH is full of loot. Full of loot. No, fuller than that. It’s a treasure trove if you know where to look. That said naturally the best loot tends to be the rarest, and there’s no rarer loot than something you planned on finding without any backup plans for if you fail to do so. As such it’s worth keeping your eyes open as you traverse the forest, but don’t count your chickens before they’ve hatched
Food: A minor thing, but are you missing some key food items, and are in the market for a few more hearts before you start the Twilight boss rush? The vast majority of the Twilight Forest consists of biomes with the FOREST tag - shocking, I know - and as such are able to spawn a wide variety of Pam’s temperate fruits and gardens. In addition the Swamp (and Firery Swamp) both qualify as places where tropical fruit trees can spawn, and rare Twilight Lake biomes naturally spawn rare Water gardens. I don’t believe you can find everything in the Twilight, but it’s not the worst place to look
Mob Drops: Something unique to GTNH is that various Twilight mobs have been given extra drops, and some of those drops make it very worthwhile to hunt down those mobs for them. One prime example is Kobolds, who have a (low) chance of dropping Arsenic dust, Alu dust, or even alu ingots outright. They are a consistent(-ish) source of alu ingots should you wish to get started with Thaumcraft. Another good example is Redcap (Sappers), who have a small chance of dropping tiny piles of Gallium dust. Check NEI for a full list, and expect some surprises in Twilight drop tables
Twilight Wells/Ruins: Scattered among the Twilight you’ll encounter the odd well, or remnant of a house that’s long collapsed. Hidden down the wells, or buried below the half-dirt floors of the ruins, you’ll rarely enounter a chest that might contain a true jackpot of loot…or a potion and a potato. Feast or famine seems to be the raison detre of these chests, but with sufficient luck (or stubborn determination) you can find void metal ingots, stainless steel ingots, and a smattering of low tier TC stuff. Expect to mostly get a bounty of various potions and the odd basic vanilla food item, though
Hedge Mazes: Hedge mazes are very basic and easy little challenges containing some spawners and some chests with very basic loot. Common drops are “rare” vanilla food items, mushrooms/pumpkin/melons and random junk, though you might find a set of shears, a diamond, or a golden apple
Hollow Hills: Hollow Hills come in three sizes - small, medium and large. They are exactly what they say on the tin, hollow hills full of (vanilla) ore up top, and swarming with mobs (and their spawners) on the bottom. The difficulty, and potential reward, of a hollow hill basically scales with it’s size. Hollow hill chests largely contain ingots of relatively low quality, things like copper and tin are common, but more valuable ingots like Alu, Electrical/Damascus Steel or Cobalt can also be present. Other rare loot includes Ironwood Pickaxes, Naga Scales, Steeleaves, Dezil’s Marshmallows, and various low level Thaumcraft-related items - primal/Balance shards and Thaumium. Finally Hollow Hills are the source of Moonworm Queens, which can be used to shoot Moonworms like a bow shoots arrows. Moonworms become a placed item where they land, acting like a torch/light source. Note that Moonworm Queens do have durability, but they can be repaired/refilled by crafting them with three Torchberries. Torchberries are a common find in caves in the Twilight Forest, or drops from Skeleton Druids which spawn at intact twilight houses (contrast Witchery houses)
Lich Towers: I’d say “raise your hand if you just pillared up straight to the Lich and never explored the actual tower”, but the mass of dirt pillars speaks for itself. More seriously Lich Towers can be deceptively dangerous places to explore, with dangerous mobs and potentially tricky terrain being the norm, but the loot that can be found there can make these explorations worth the effort. The most common loot from Lich towers is masses of books from the plentiful bookcases found there, handy if you want to build a library, as well as basic potions and other low quality stuff. The good, rare loot includes Dezil’s Marshmallows, Stainless Steel ingots, Eyes of Ender (normally requires an HV tier chemical reactor, or a trade with a Priest villager - extremely key for magic progression), Tiny piles of Nether Star Dust (basically a Nether Star if you’re able to dupe them through Null Catalyst), and a Peacock Feather Fan. Peacock Feather Fans will conjure a blast of air that pushes away nearby mobs when used on the ground, or launch the player a dozen-odd blocks up in the air if used while in the air, serving as a kind of double-jump. In the latter case you will also get a second or two of Jump Boost, so that you won’t take (as much) fall damage when you land. Peacock Feather Fans will consume durability if used for it’s mob pushing ability, which you’re most likely to use accidentally TBH, but it can be repaired in a Thaumic Restorer
Minoshroom Mazes: Mazes in the swaps are not the most profitable of places to comb through, but there are some useful things to find. Common loot largely consists of some vanilla stuff and materials useful for the ill-prepared maze explorer - paper and leather is common, with regular golden apples being a less common sight. More useful finds include Ironwood and Stealeaf (either directly in the form of tools/armor), Maze map foci, and an uncannily high amount of Charms of Keeping. Note that Charms of Keeping are not exclusive to the maze, but they seem relatively common inside a maze, no doubt made even more common by Maze Slime mobs. Note that the maze has a secret treasure vault on the bottom floor, and if you manage to reach it without blowing it up you’ll be rewarded with some enchanted gear, emeralds, potions, and possible a Mazebreaker. The Mazebreaker is an otherwise unobtainable pickaxe that has the special quality that it doesn’t take extra durability damage from breaking the various maze blocks, which is what normally is supposed to discourage players from cheesing the maze. Hint: Equal Trade is a great wand focus
Knightly Stronghold: These places seem to largely share loot tables with mazes, only with maze map foci replaced with knightmetal (shards), and a more even spread of Charms of Life/Keeping. Not the best loot all told, but you’re likely here for Knightmetal anyhow, so a free meal is a free meal. Defeating the boss will yield some enchanted Knightly and Phantom tool/armor pieces, which likely don’t serve much purpose beyond decoration, if any
Ur-Ghast Tower: The Ur-Ghast tower is an odd duck, loot-wise. The vast majority of it’s loot chests seem unchanged, likely due to their part in progressing through the dungeon (if you know what that’s referring to, congrats). The odd chest that has modified loot drops low level Thaumcraft stuff and Stainless Steel, suggesting sharing a loot table with Twilight Wells/Ruins, though this is just a guess. What is for certain is that Ur-Ghast Towers are treasure troves of Ironwood/Steeleaf, if you didn’t have enough for all of your soft mallet needs. Beyond that the biggest loot in the tower is likely the tower itself - the odd brewing stand, bookshelves, etc. - and in one of the section you will find Carmanite Ghastling spawners. If you wish to set up an early Ghast spawner for Ghast Tears, this is likely the best mob to farm. Certainly the easiest, given the readily(-ish) available spawners for them
Snow Queen Tower: The snow queen’s lair is (or at least feels) unfinished, but there are loot chests hidden around the various rooms. That said you’re not likely to find anything you can’t find/get more easily elsewhere than a few unique bows/sword (possibly a drop from defeating the Snow Queen), and spoiler: You probably already have something better by now. The main draw of the Snow Queen’s tower would be the Aurora blocks that make up the tower itself, as they can be electrolysed into helium and helium-3, except that you can get both (and cheaper) from Endstone Dust. Which you are all but guaranteed to set up by EV at the latest, and helium has relatively few uses in the meantime. That said there are paths, however bizarre, where taking an Equal Trade focus to a snow queen tower can set someone up with a powerful source of early helium
Lamp of Cinders: Something GTNH specifically adds is a secret quest reward for completing the Twilight progression up to the Snow Queen, then going the extra mile to get a Lamp of Cinders from the Troll Caves. Sufficed to say that if you’re short on mob heads this might be worth pursuing…
Mushroom Tower: An unfinished structure found in Deep Mushroom Forest biomes, these gigantic towers are entirely empty save for some oak planks and worrying amount of mushrooms. If ever you want a manual mushroom farm, look no further. The cap alone is 16 blocks of solid mushroom
Twilight Oak Dungeons: A hidden secret even I have little familiarity with, and I know the “intended” way to fight the Ur-Ghast. These dungeons are hidden rooms inside some Twilight Oak trees, containing a spawner for a small spider varient and a chest possibly containing one of the special saplings. Having literally never found or used any of them before I’ve zero clue what those special trees do if grown, or how useful they are(n’t) in the context of GTNH. If nothing else you might impress a friend with one, and your seemingly bottomless knowledge of the Twilight Forest
Questing Ram/Rainbow Forest: A rare find in the Twilight, giving the Questing Ram all 16 colors of wool blocks will reward you with blocks of vanilla materials (not really worth mentioning in GTNH) and a Crumble Horn. Using the horn will rapidly break local stone into cobble, cobble into gravel, and break gravel and dirt as if broken with a shovel. The range isn’t great and the item does have durability, but it’s a neat thing. What practical use it has I’m not sure, but honestly I’m assuming that in GTNH, by the time you can get one, it’s merely an interesting trophy