Overview

Entity 500, better known as Blood Marrow, is a deadly yet elusive type of entity that is most commonly found throughout higher-numbered positive levels of the Backrooms and is more often than not encased within surreal formations made of bone. Not much is known about the entity itself due to the danger it poses to any nearby human life and its tendency to stay within its bone cocoons for long periods.

While much more deadly when out of its bone cocoons, Blood Marrow nonetheless inflicts nearby wanderers with various anomalous effects when in said cocoons. It has been speculated that the bone cocoons themselves give the entity the ability to exert these effects on wanderers, though as no one has thus far survived a confrontation with the Blood Marrow itself, this is unconfirmed.

Behaviour

Blood Marrow is believed to be highly aggressive when outside of its cocoons of bone; indeed, those who have seen it themselves perished soon after. Sometimes, it will even attack entities, though most cases of this are when the Blood Marrow cannot find a human target for a long time. When attacking, Entity 500 morphs its body to kill its prey either via brute force or by creating sharpened tendrils out of its substance to cut and impale prey. Those who have been directly attacked by the entity itself have so far never survived the encounter for longer than five minutes, though the exact times cannot be specified for certain.

Any instance of Blood Marrow behaves substantially differently when in its bone cocoon. When one is near a still-inhabited cocoon, one may experience an unwillingness to leave the area the cocoon is in, as well as carelessness regarding any anomalous effects one may be experiencing, though one can usually still easily escape within the first few minutes of being there. Moreover, one may begin to have strange delusions or hallucinations, as well as a possibly more lethal effect: on occasion, when one stays near a bone cocoon for too long, their skeletal structure will start to move and jolt within their body, causing severe pain and death. On even rarer occasions, the bones in the victim's body may subsequently grow and form a formation similar to that of the bone cocoon, absorbing the rest of the body in the process.

Biology

Blood Marrow is speculated to be made of congealed blood, based on various witness reports; however, the biology of the entity is completely unknown. Nevertheless, the bone cocoons that instances of Blood Marrow reside in are confirmed to be made of the bone of whichever creature it initially grew from — this process is explained in detail below.

The Growth Cycle

When an instance of Blood Marrow attacks a living creature, it will typically infect them with a mysterious supernatural disease that, over time, will cause their bones to deform and grow unnaturally, eventually killing the victim. Over about 72 hours, their mutated skeleton, with the flesh now having been absorbed and used as fuel for the growth, will have fully transformed into a Blood Marrow cocoon. This bone cocoon will quickly begin to grow new Blood Marrow inside of it^[1]. Eventually, after an indeterminate amount of time, the Blood Marrow will break out of the cocoon — usually only when there are potential victims nearby — and the bone cocoon will slowly rot and crumble to dust. Thus, the cycle continues. Meanwhile, the Blood Marrow that initially inflicted this 'curse' is presumed to roam rampant throughout the Backrooms^[2].