Tenebrous Depths / Tenebrity

This is a biome/dimension only accessible by falling into the void, primarily acting as a consequence for breaking the games boundaries. This is where the sculk seeped through the bedrock and into the ancient cities. Echo crystals can be found there. Echo crystals used to be another strong mineral, but ultimately have succumbed to the sculk. Breaking the crystals gives you echo shards, which can be used to craft echo powder, Arrows of Resonance (made with echo powder, they mark a few entities wherever the arrow is shot), Echo Heart (Heart of the Sea (or something else) surrounded with shards, can be put in a beacon as a secret item which doesn't show on the menu which hides status effects, provides a new effect called Silence, and makes the beam of a beacon translucent/transparent), Sculk Table (crafted with a Crafting Table and four shards, this is the only way to craft these items, for they are inaccessible in regular crafting tables), Sculk Helmet (crafted with two Sculk Sensors and three sculk blocks. It gives blindness, but whenever there is noise from an entity, they can be seen through walls), and maybe more. You can find regular Deep Dark stuff down there, but there are two new creature which can be found, being the Watcher (similar to the phantom, keeping distance but alerts sculk to your presence. It drops a sculk shrieker upon death) and the Sculk Tentacle (can grab you, squeeze you, wack you, and kill you). I'm running out of space, but it's some food for thought.