- Cavern world, where the entire world is based underground.
- No NPC challenge, pretty self-explanatory.
- 1 HP hardcore FTW, master mode (already attempted by many, borderline impossible)
- Grapple challenge, where you can only use grappling hooks to move.
- Hardmode at spawn, where you begin in hardmode with nothing.
- Hellscape, where the entire world is Hell, excluding the surface, which is made of meteorite ore.
- Constant night challenge, where the world never turns to day
- Extra gravity challenge, where you jump 1/2 as high and move 2x as slow.
- One damage challenge, where every weapon has one damage.
uh that's all I got
Not sure if you can set such things like the entire map would be underground of such. - you can do the drow challange though, like, just simply go underground after you spawn and stay underground. I tried it (pre-hardmode), and it's not that bad. but you lack a few common resources like wood and fallen stars.
What I wonder about more is the "hardmode from the start" challange. I did thought about it, and I wonder how difficult it actually is...?
Thoughts about "hardmode at the start" challange (didn't tried yet, but maybe I should give it a try?)
if you have a normal purity forest spawn, you won't have a super hard start, since there are only slimes, right? most of the surface is not that bad. forest definitelly shouldn't be a problem at daytime. (but you wouldn't want to loiter around there the night at the start)
The tough surface biomes are the jungle and the evil biome (which will rapidly spread, so that can be a huge problem too, it really depends on how the world is generated - protecting the spawn area with trenches might be needed relatively early.)
normal early game weapons and armor would be mostly useless. but the bound wizard can be found from the start. so, you could try to get some books from the dungeon or a bookscase if you find one in an underground cabin, and gather soul of night and ichor/cursed flames for a great weapon, relatively early game (of course, surviving in the hardmode evil biome would be much tougher than that. you also have access to the crystal ball for both clairvoyance buff, and as a crafting station, for things like endless arrows/musket balls.
mining for most stuff would mostly be only used for deco, metal armors would be even more useless than usual. however, you can try to get the early hardmode metals since really early game, either via fishing - it would be tedious, but a great way to aquire some hardmode ores from the point you have a good fishing rod and decent gear. catching pearlwood crates shouldn't be that impossible, so, you might can aquire a cobalt or palladium armor as your first real armor.
of course, if not via fishing, you could go for the altars too, if you don't mind the pirates and mech bosses attacking on their own. with a surface mushroom biome, the truffle should move in, and he sells hammush, which should have enough hammer power to smash altars.
most monsters would be very tough first, especially because of the ver limited mobility, but you can get some really awesome drops with luck, both weapons, and other gadgets, such as ankh charm components and such.
sounds tough to fight hardmode monsters with pre-hardmode gear, but you could gear up to cobalt/palladium tier with some fishing and luck, I think, relatively quickly, and then you're not much different from early hadmode on usual playthroughs. you also might get a few quite strong weapons drop early like an amarok, a beam sword or such. things that are somewhat negligable on the usual playthrough, but would boast with great power on this.
interesting that (if hardmode is set via T-edit) I think there would be no Hallow, since that spawns when WoF is defeated. so, an interesting question thether it would spawn, once you defeat WoF, or not? You would be able to buy hallowed seeds from the dryad since the time she moves in, so you could easily create an artifical hallow biome too.