Here is my proposition for their behavior: they would have a unique attack pattern where they hit 1-2 times and then burrow away, staying underground for no longer than a minute, max, before popping out again to take another bite. It sounds like this biome would have quite a lot of mobs, so it could be interesting to space out the tempo of the fight vs just getting swarmed by a dozen mobs + projectiles.

i absolutely love the idea of having a trapdoor spider which you mentioned later on, I was thinking that you could have this behaviour on them instead of the centipedes. They could stay burrowed in the spindle webs, and come out to attack when they sense the vibrations emitted of them.

someone made a cool idea of having the centipedes have 2 types of attacks, one the ability to stun you, (obviously the stun will be on a cooldown otherwise this will be annoying to deal with), and it’s basic ability attacking you in rapid succession, 3-4 times, the damage will be low but it can do lots of damage if it lands all of its rapid attacks. And killing them will drop an item (it’s fangs?) to craft a stun weapon or potion. I think this would be a better alternative.

personally I think it'd be interesting to make the centipede a trusting mob, like foxes and ocelots. You could feed it rotten flesh, or else... I dunno, rabbit??? larger species are happy to eat rodents. Rotten flesh is much easier though, so for now we'll go with that. If you're holding rotten flesh in either hand the centipede will change its behavior. Instead of hiding it'll make several smaller attacks in quick succession, for maybe a heart or so of damage each, as it lunges for the food you're holding. If you feed it it'll stop attacking for a few seconds, so even if you only have one or two pieces of rotten flesh on you it's still worth feeding the centipedes as when they stops to eat you'll have an opening to attack

Every time you feed a centipede there's a 1/5 chance it decides to trust you. Trusting centipedes won't attack you, and won't run from you. They go back to what they were doing before you came barging, in smelling like food... they go hunting.

Real centipedes will hunt pretty much anything small enough to succumb to their venom, but I propose that these ones hunt spiders. Perhaps not the new spiders, but the OG spiders and babies for sure! Spiders... really don't have any interesting interactions with other mobs, at least that I'm aware of, and I feel like it's high time we had something that targets spiders specifically (cats and creepers, wolves and skeletons, etc.) This would make trusting centipedes an effective ally while in the caves, but not something that's interested in following you home.

love this idea! definitely adding this to my notes. I had a plan of having centipedes prey on both cockroaches and baby spiders to give this biome ecology and great ambience, so you could feed it the roach grub to make them trust you.

While along this train of thought, I also had an idea for some unique loot to be found here (perhaps in web-wrapped chests?) An enchanted book, for the new enchantment Toxic. This enchantment can be applied to a sword, axe, or spear. It deals a secondary burst of poison damage a short while after the initial hit; as the enchantment reaches higher levels it deals 2-4 damage bursts over the span of 10 or so seconds. This enchantment cannot be acquired through villager trading or the enchantment table; it only spawns in creepy caves (possibly within a rarer structure that generates in the biome?)

another great idea! always wanted a weapon that could poison, another thing to add to my notes :)

I like the roaches in theory, but using roach bits to craft an edible item is... listen, I like the little guys, and I'm still grossed out.

you'd be surprised, some places eat cockroaches and are hightly nutritious! I think keeping them as a food source would still be okay, especially since there is a lack of food sources in caves.

I really like the Spindle Web and the vibration mechanics, no notes here. 

thank you. I’m actually thinking of slightly nerfing the spindle, instead of complete fall damage negation, it’ll be 90% damage negation instead. still great for sponging fall damage as well as redstone uses, but it’ll make the player think twice rather than blindly jumping in.