Hollow World

The "Otherworld" is the main setting of huke's artwork, and the secondary setting of the story of the Black★Rock Shooter OVA and the Black★Rock Shooter anime. Black★Rock Shooter and other characters live in this realm, and the bizarre landscape is a witness to constant fighting between its denizens.

The true name for this realm is not yet disclosed, and to aid in reference, will be called "Otherworld". It is possible that this is the world of "Hazama" in the Innocent Soul manga.

In the Black★Rock Shooter anime, it is heavily implied to be the "world of many colors" mentioned in Li'l Bird Li'l Bird Colorful Colors.

Black★Gold Saw's Battlefield
It is only shown to be a large expanse of land covered by grotesque features of land surrounded by an endless sea. From inside, it is composed of a variety of structures, with most buildings bearing a dilapidated, post-apocalyptic, and even desolate look to them. A striking feature of the structures erected in this realm is its dominant theme of checkered tiles in most of the floors and walls depicted.

Black★Gold Saw and Black★Rock Shooter fought here in the beginning of the OVA, where the former was victorious against the latter.

Dead Master's Lair
Dead Master stays inside a structure similar to a cathedral complex located in the middle of a large depression, surrounded with winding passageways from outside. The area is surrounded by a large cemetery, which is shrouded in a thick mist.

Desert Area
A vast expanse of dry land with towering spires of earth and rock eroded over time. Strength was seen here.

Sky Gates
Black★Rock Shooter starts here at the beginning of the anime. The area matches the decor of the Otherworld in general and has pointed, deserted stones everywhere.

Chariot's Land
Chariot seems to reside here. The area is full of robotic, toy-like creatures that have claws and drills. The background resembles a toy area with baby doll faces and colored blocks.

Inside is a house that floats in the air, suspended by strange string-like objects. Dead Master was dragged here by what was presumably Chariot's minion bugs and chained. Only a single doll covered this up.

Chariot and Black★Rock Shooter fought here until Black★Gold Saw intervened and made a fissure, separating Dead Master from Black★Rock Shooter.

Black★Gold Saw's Battlefield
In the anime, it seems to have ruined Greek-style columns, and multiple chains spread around.

Black★Gold Saw created the strange hooded figures here.

Dead Master's Lair
The strange place where Dead Master was chained might be her lair. In that place, there lies a hidden throne where the Dead Master sits for the duration of time while Black★Rock Shooter fought her skeletal minions. Skeletal colums and Greek-style pillars are scattered about and the entire battlefield has a green tinge.

Covering the floor of some parts of the battlefield is a strange, greenish liquid.

Unknown Battlefield
Until Dead Master was killed by Black★Rock Shooter, this area was Dead Master's lair. The trauma of killing Dead Master transformed Black★Rock Shooter into Insane Black★Rock Shooter, as well as transforming the area into a purple-tinted area filled with spikes and cannon-like objects, possibly meaning this area belongs to Insane Black★Rock Shooter. This is supported by the fact that the area bears an uncanny resemblance to the normal Black★Rock Shooter's realm.

Black★Gold Saw is shown fighting with Insane Black★Rock Shooter here, eventually moving their fight to Black★Gold Saw's own battlefield.

Strength's Battlefield
Strength's field is like that of an orange rubix cube surrounded in gigantic barbed wire. She can easily change her position on the cube by slamming her fists in one of the blocks. She can easily move through the field from jumping or climbing from block to block. It also seems to react to her emotions for when she starts to go insane the field is engulfed in flames.

After saving Black★Gold Saw in her own battlefield, Strength fought Insane Black★Rock Shooter here.

Black★Rock Shooter's Battlefield
According to Mato and Insane Black★Rock Shooter, this is Black★Rock Shooter's world. However, she is never shown to actually reside here. The world is a wide, empty space with some ruins embedded in the floor. The ground has the typical checkerboard pattern of most locations in the Otherworld, but with a blueish tinge. The entire battlefield seems to be surrounded by a black-blue rocky material that prevents those inside it from seeing the outside world. This battelfield resembles a cavern or a large hall. Unlike most of the other battlefields, it is practically empty except for some large, spiky objects stuck in the ground.

Despite Black★Rock Shooter transforming into Insane Black★Rock Shooter, Mato finds a ★rock Cannon embedded in the ground here.

Mato Kuroi (as Black★Rock Shooter) fights Insane Black★Rock Shooter here. During the battle, stars begin to appear in the "sky" of the battlefield and eventually the worlds of all the different "other selves" combine with the battlefield, allowing Mato to summon a giant ★rock Cannon, which she uses to defeat Insane Black★Rock Shooter, and destroy the wall of the battlefield, revealing white space behind it.

Trivia

 * While in the 2012 anime, the Otherworld is primarily referred to in terms of "the other world" or "that other world", the website occasionally uses the term "imaginary world" (虚の世界 kyo no sekai). However, since it always appears in quotes, it can be interpreted as a temporary name, and thus the true name of this world remains undetermined.
 * It can be implied that the design of each world represents a certain emotion that a certain main character is trapped in. For example, Strength's world represented by a rubik's cube in motion of being solved, impling one being trapped and unable to escape. Black★Gold Saw's world implies desperation and sadness, Chariot's world implies desire and stubborness, and Dead Master's world implies despair and rejection. Black★Rock Shooter's world while serving as an antithesis represents a blinded view as well as the concept of absorbing all the negative emotions.
 * The merging of the worlds as seen in the anime represents the absorbing and integrating of all emotions into one world, hence giving it completion.