The Quantum Realm is one of many realms in the RBM universe. This realm is exceptionally powerful, however, being able to amp up anyone who resides in it for too long. This rise in power is known as converting a human into a Quantum Beast.
As previously stated, if one remains in the Quantum Realm for long enough, they will transform into a Quantum Beast. The amount of time it takes for a person to become a Quantum Beast ranges from just a few months to over several decades. The length of a transformation is completely random, so there's no way to alter one's self to speed up the process. If a baby is born in the Quantum Realm, they are immediately a Quantum Beast.
There are few known entrances to the realm. Most are secluded portals in empty or undisturbed areas of the universe. Due to this, around 99.9% of the portals are found under the sea, making the ones found on land to be extremely rare. Most portals are also almost indistinguishable from the rest of the world, so most people who find it do so on accident.
A Quantum Beast, however, can enter and leave the realm at will via teleportation. This proves very useful in tight situations. (For example, Quarzone takes Trant to the Quantum Realm at the end of Hell's Front Door.)