These aren't sorted by difficulty. ONLY listing the custom structures important for advancements, gear, enchants, quests...
Please try to explore most of them before not doing so because of the loot. The term "Useful junk" means many things. It can mean both wind charges, golden carrots... so on, but also common armor trims, building blocks, vines.... There's no "one-word" term to use for it.
Hidden in the underground lies one trial chamber-like room.
Doesn't feature any of the custom enchants.
A flying island that you get into by looting a part of the asflors village.
Features an ominous traveler key for opening the asflors village ominous vault, as well as a secret entry to the runic dimension...
Beginner level loot and mobs.
Shrines
They go by tiers (Tier VI on the picture), and from Tier V up, you can get the ominous vaults. They are "smaller trial chambers"
Overgrown set of buildings all full of creakings and zombie villagers
Structure full of strays and their arrows... At the end you can find a map to the lone citadel
A structure full of strays and their arrows, loot suitable for beginners
A village full of witches
The illager structure that lies underground. Has usual pillager loot, also features zombie villagers you can cure for an advancement, but they sell the illager's bane enchantment.
One of the largest structures in game, has a lot of mobs, the loot includes various things, most of them for beginners. Also features the Might book.
They have many types and they are in every biome except the ocean. The cartographer inside can give you a map to a few structures. (on the right you can see a part of another structure: Asgard)
These gargantuan trees are a test of strenght before entering the Runic dimension, their main loot is enchanted books, shulkers and some other useful junk. From the vaults you can rarely get spawn eggs: witch, iron golem, breeze.
All around the world, they include one divine key trader and a map to the structure they belong to.
Has a few zombies, then drops beginner-level loot, and also some mellohi, cat, blocks, chirp and strad disks.
Has a lot of illagers inside. You could even say it's a bigger mansion. The loot is mostly consisted of totems.
Overgrown cave systems with non-difficult mobs and with loot for beginners.
It is a very peaceful village. The loot is made mainly out of enchanted books, but also other useful junk. By reading the story, you'll get to the Runic fracture and later discover where the Sword of Asflors is.
A structure found in swamps. Witches use it as their luxurious resort, where they make potions with many ingredients which you can find there. Antidote enchanted chestplate is recommended to use here.
Everything you have gotten from previously looted structures needs to be used here. It has many strong mobs, but features many things: Gungnir, Zyask's strike ,spawn eggs (depending if you have the bad omen effect or not, you get): normal: enderman, zoglin, zombie, pig, Asgard guardian (zombie version) Ominous: zombie, husk, wither skeleton, Asgard guardian (Both wither skeleton and husk versions), evoker, piglin brute. From the normal vault, you can get enchanted books, and spawn eggs: cat, wolf, and RARELY, you can get a trial spawner.
If you manage to find the ominous helheim vault, you are able to get Nugrak's bite and in some cases even monster spawners..
Has a lot of mobs, the loot is diamonds and some other useful junk, but most importantly it features the Gea's gift book. It has a few types/parts: deep dark part, nether part and the pale part. If you stumble upon the pale part, you can get protection V leggings as well (Warning: rare).
Has a lot of gold, iron and TNT inside. It is filled with pillagers, vindicators and other illagers.
This is the lair of the witches, it's filled with spiders, bogged and the stench of poison. Features both Blowthrough and Tri-shot books.
Just as useful as an ordinary witch hut
These buildings are filled with sand and a few traps too. Features an Outreach enchantment book.
A structure filled with blazes. Can have wither coated books.
A structure filled with piglins and their wealth
Can have maps to other nether structures
A giant structure filled with piglins and their wealth, has a lot of netherite.
A structure filled with piglins and their wealth
"Giant nether fortress", has a lot of nether-related loot
Can have maps leading to other structures and wither skeleton heads.
Can have maps to other nether structures
A monstrous structure filled with shulkers and phantoms, features many end crystals, end city loot and gravity books. Inside the ominous vaults, you can get an elytra.
You can get end city loot and an elytra
You can get many shulker boxes and end city loot, along with a map for the end castle
Mobs are a bit more difficult than they are on the Alfheim tree, but the loot is much better and the rooms have way more strategy needed to loot them. The ominous vault can drop many things including netherite, blaze, mooshroom spawn eggs, heavy cores, or the Twilight of Yggdrasil bow.
Per each labyrinth, there's one Dark Elven bow hidden away to be found.
You start by getting full protection IV netherite armor. Then you can move on to getting the Asflors sword, by finding and exploring the Asflors village and finding the necessary books to guide you on the way to the main loot.
After getting the sword, you can test your strenght in the Alfheim tree. If you trust yourself that you can loot it with ease, go to the runic labyrinth, featuring many more mobs and two more items to get: The Twillight of Yggdrasil bow and the Elven Bow.
When you get those, you move on to the overworld, now you need to get spiteful 3 on your sword and probably traveler 3 on your boots, you need to explore to find those, they're not 100% guaranteed anywhere.
Then you explore and find the Echoing forge, where you get the Gea's gift II enchantment. Get a full set with it.
While within the Echoing forge, you might get protection V pants from the forge's pale part chests. You can skip this step, but it's safer to have a larger protection than not.
Now it's time for the hard part, going to loot Asgard, where you can get Gungnir, but you can also get your gear lost very easily, proceed with caution. You can also get Nugrak's bite and Zyask's strike within the structure.