The Between-space is the dimension where the Sinmur Stones are. It is designed to seem like islands in the Void.
It is accessed with a Charged Withus Warp.
The Between-space is formed of Voidstone. It is generated as a series of large, wide islands starting at y=1 and extending to y=64. There is a ceiling of bedrock at y=256.
Corruptinium Crystal Ore generates in the Voidstone. This can be used to farm Corruptinium Crystal, but it is very dangerous and can easily knock players off of the islands.
The Between-space's sky is entirely black, with no stars whatsoever. It also constantly generates void particles.
In the Between-space, Jump Boost IV is granted to the player. When the player is standing with no Voidstone at most 40 blocks beneath them, they will be given Nausea II.
When the player is in the Between-space, Minecraft's ambient cave sounds are played to them from time to time.
The only mobs that spawn in the Between-space are Void Phantoms, so fighting isn't a problem.