Quick answer
And these are just the minimum distances, so you may have to travel way farther than that. I canvassed the internet, Google searched, AI summarized, deep dove into Reddit, and even asked my own community to find the craziest and sometime useful seeds that you need to try in Valheim. You can even add a seed to the world if you have one, or let the game randomize it for you. On every Valheim world seed, there are 10 possible locations each for Haldor, Hilde, and the Bogwitch.
Both onion and turnup seeds are commonly difficult to find. Carrot seeds can be a bit difficult to find since they are not very common, only spawning in this biome. The Mistlands, for example, doesn't enter into world generation until about 6,000 m from the center. And to start things off, this seed probably has the craziest spawn I've ever seen, shared to me by Barefoot Cookie.
Onion seeds, for instance, can only be found in chests across the mountains overworld and come with a tiny loot chance of a mere 12 1/2% chance. the wild carrot seed, distinguished by their white flowery tops, tiny and assuming and about to completely change your diet. So, it will be a while before you find its alt biomes. In most seeds in Valheim, you start very close to the world center, but this one flips that on its head.
Seed basics
Turnup seeds, on the other hand, are not found in a chest, but rather growing in the swamp's biome. The good news is the Black Forest has everything you need to start producing bronze, so you can get your farm started pretty quickly, and you definitely should once you found carrot seeds. And you can find these as early as 500 m from spawn. But for starters, we'll begin much closer to home with the Meadows, which has nine possible variations like the raspberry, dandelion, and mushroom that has a massive density of those resources.
Here, you'll find yourself nearly 2,000 m from it, which has all kinds of complicating implications. So, admittedly, all the attributes of the seed are negative and probably only useful for a challenge style run. With the swamp being covered with so much water, the turnup seeds spawn far less frequently than carrots in the Black Forest for comparison. The mushroom variant, specifically, can also be found in the Black Forest.
To make matters worse, the three Eikthyr locations are nowhere near you, occurring again almost 2,000 m away. Even though it's the only seed I've ever seen remotely like it, so I had to share. And this seed gives you four chances for it in your spawn. What's really interesting about these alt biomes is that many of them can overlap to make really wacky combinations.
How to use seeds
In fact, the Elder, Bonemass, and even Moder are all closer to your spawn than even Eikthyr. As an alternative, this seed has one instance of each wood house two and six even closer to spawn with backup locations just a little further out than the previous seed. You can also encounter this alt biome type in the Plains. At the same distance of 500 m, you could also encounter a birch or small tree Meadows.
Another resource whose spawn can be suspect and always needed in huge supply is iron, and sometimes you can get some bad seed luck with swamps being far away or containing limited sunken crypts. Next, at 1,000 m, you can find peaceful Meadows that will likely make for the optimal base location. The birch, like previously, will consist of a normal Meadows, but with way more birch trees than normal, while small tree will be absolutely covered in saplings. Optional, but not necessary, you can add railings to the side to help keep the boar on the ramp.
None have Eikthyr, any wooden structures, almost nothing to speak of. You can use any material you want as long as you're covering the ground below where the newborns will fall. In this case, I found an absolute gold mine in a Meadows instance that is composed of both a birch, small tree, raspberry, mushrooms, and dandelions all in one. This seed gives you the best chance to find both early, having four of the wood house two structures and two of the six variant extremely close to spawn.
Best seed choices
I imagine these are incredibly rare, but considering how many players complain about not being able to plant mushrooms and berries, finding these and placing a portal there would basically solve their problems. Further from the world center, the alt biomes get more interesting at 750 m with the menhir alt biome. These add a series of almost horn-shaped bones and supposedly add spawning skeletons, but I didn't happen to see any. Not the case in this seed.
What you could do is use this seed as your iron mine world. However, the indication is subtle that you're in one. Even make multiple worlds with the same seed to solve all your iron needs. Keep an eye out for blue fireflies in the air, more easily spotted at night, and you might notice a higher density of deer and boar.
Not to mention the second largest possible mountain that can be found in any Valheim seed, commonly known as the hook, is located on the next large landmass over far to the north. What's special about this one is that it blocks 11 other hostile creatures from spawning. Of course, you will still need to sail to both the north and ashlands, but that's the case in all seeds. It's not clear which creatures it blocks, but I imagine it prevents the more dangerous night spawning creatures that begin to appear in the meadows as you progress the game defeating bosses.
Map tools
Another seed to consider is this one, which has all three trader locations practically on top of each other. And lastly, there's also a white deer in the game files that I spawned in. Of all the seeds I researched for this video, no other seed has all three so close together. Beyond that, occurring at 1900 m are the dark meadows.
However, it's important to remember that if you try and use this seed for this purpose, that you stay away from any other instance of where the vendors can't spawn. Here you'll find Draugr villages predominantly on lower terrain and have more hostile weather conditions. The reason being that on any given seed, each vendor has several possible spawn locations, and the second you find one, all others become invalid. The only other alt biome reference in the game files is one known as lantern which states it could occur in any biome from meadows to mistlands, but I couldn't find a single one.
If you're desperate to find Tetra, which I know can be hard at times, try the seed kitchen sink. I was able to spawn in this lantern structure that I can only assume was intended to be found in these alt biomes and used as a beacon for the deep north you eventually travel to. While its starting island is still quite big, it's made notable by much wider expanses of ocean than your average seed. Again, like the white deer, this could be a feature cut from the final delivery or maybe a bug not yet patched in the press version I played.
Tips
In Valheim, there is a master seed in which all other seeds are derived. The black forest, like the meadows, has nine possible variations including many of my personal favorites. Effectively, each time you enter a random sequence of numbers and/or letters to generate a new world, a circular slice is taken from this master seed. Starting at 500 m you can encounter locations of resources like we saw in the meadows, but this case massive blueberry patches and dense pockets of red mushrooms.
And again, this is an oversimplification of the process, but can explain why you may encounter similar shaped landmasses on completely different and unique seeds. Also at 500 m are the kallig patches which take the typical black forest and strip it of all trees and ramp up the number of stones you can find. Included in this master seed are three mountain peaks that reside far higher than any other and allow a Viking to climb above the clouds and in the case of the tallest, nearly 500 m. The one I found happened to also be covered in blueberries.
So, when you generate a new world, if your slice of the master seed happens to be taken near any of these three spots, you'll have a massive mountain to climb and perhaps build a base. Best of them all at this distance are ruined black forest. Thorn can be found in this seed and super close to spawn just in the northeast and nearly consists entirely of its own island, too. Here you'll find some unique stone structures scattered about, but more mysterious are these green wisps.