The sistrini /sɪsˈtrini/ is a delicate, aquatic stranger with white skin and thin blue stripes that spiral around its body from its nose to the tip of its tail. The exact shade of these stripes varies regionally, with cyan, navy, blue-grey, and aquamarine being the most common shades.

Lacking legs, the sistrini instead, possesses blue-colored tendrils, which it keeps close to its side as it swims. It is capable of growing and receding these appendages from its body as needed, with older individuals possessing more tendrils. The flesh is light-colored, and much softer than human tissues, offering little resistance to being cut or torn. Its jaws contain row after row of jagged, uneven teeth, although the sistrini rarely opens its mouth wide enough to show them.^ƈ Its mouth does not connect to its internal body cavity, which is filled with a bright blue liquid composed of hydrogen cyanide (80%), chlorine (15%), and sulfur mustard (5%).

Injuries tend to prove fatal to the sistrini, with the flesh softening and growing ragged around the point of injury, and the caustic inner liquid seeping out into the surrounding water.

The sistrini appears in secluded underwater locations. It is mostly found in enclosed pockets of water^ɮ deep underground, but also appears in man-made structures such as large pipes, reservoirs, and underwater cooling tanks. It appears to slightly prefer mid-sized, spheroid chambers over thin pipes and cavernous, irregular spaces, and never appears in any space occupied by fish, plants, or other life forms.

Exact duration of development varies regionally; it can take anywhere from six days to several decades for the sistrini to reach maturity. Only 64% of sistrini develop fully, however; these "unfinished" sistrini dissolve away into chalky water once their development ceases.

The sistrini's demeanor is graceful and mesmeric, and it spends its life swimming in circles with twining, hypnotic motions. Although its weak body makes it vulnerable to snags or tears, it always swims in such a way so as to avoid contact with any solid surface. Groups of sistrini spiral around one another with coordinated, ribboning motions. The sistrini generally stays within one specific chamber within its environment, and when no single large chamber exists, it travels through its territory in consistent circuits.

As the sistrini ages, its body grows weaker, and its songs grow quieter. Its tendrils no longer curl and twine around it, but instead hang limply by its side. In this way, it dies a slow, prolonged death over hundreds of years, and when it does finally pass on, it does so with a placid carol, one final swan song heard to all sensitives above. Upon its death, the caustic chemicals in its body grow instantly inert, as its flesh breaks apart, first to white, silky sheets, and then, into milky fluid that dissolves into the water all around it.