The simdroni /ˈsɪmdroʊni/ is a black, narrow-striped stranger with slender, pinkish hands and feet. It is smooth and somewhat warm to the touch, and a faint vibration can be felt if pressed upon firmly.^Ⴗ The internal composition is a uniform, rubber-foamy flesh that is pure black in color. When either the skin or flesh is cut, though the simdroni does not bleed, four percent of individuals do expel a soft, nearly invisible vapor. Regeneration is slow, taking up to ten weeks for even a thin cut to heal. Although the simdroni does not scar, the white stripes are slow to reappear across once-damaged areas.

In accordance with its reticent, subtle nature, the simdroni is odorless, lacks a mouth, and makes no sounds at all. Further, its body displays sound-dampening properties within a range of three inches, allowing it to move silently even over carpet or nearly any other rough^Ⴁ surface.

The simdroni generates exclusively in single-person homes. Closed curtains, dark lighting, and quiet surroundings significantly^ꝲ raise the likelihood of a simdroni infestation, with frequent visitors or other social activities (such as phone calls or text-based communication) decreasing this likelihood.

Within these spaces, the simdroni appears beneath couches, under tables, or behind bookcases, though the exact moment of generation cannot be observed. When it first appears, it is thin, and small enough to fit in the palm of one's hand. Growth is gradual; it may take up to ten years for a simdroni to reach full size.

The simdroni possesses a muted, subtle disposition. It spends its days at rest, and although able to move, it remains relatively sluggish and inactive during nearly all times. Any interactions with its environment are rare and random – every few weeks, for example, it might look at several book titles,^ꝸ turn a dining plate several degrees, or bury its fingers into the dark grey fibers of the bedroom carpeting.

The simdroni dislikes noise of any kind, and can even be weakened and killed by loud enough sounds. Sudden bangs, crashes, or the sound of a television being knocked over cause it to cower and raise its arms in a protective gesture, although it does not tremble, or show any other sign of overt fear. Due to this aversion to audio, the stranger, when able, silences any noise-producing objects within its territory, including radios, ticking clocks and even humming appliances. This destruction is not performed with malice or savagery – instead, the simdroni disables these objects with a careful, clinical precision.

Aside from these behaviours, the stranger's main preference is to silently observe the person with whom it co-habitates.

The simdroni's presence atrophies communicative abilities in its victim, with this ability affecting both sensitives and non-sensitives alike. While at first, its victim presents as relatively normal outside the home, as time goes on, they begin to spend more and more time alone,^Ͼ and these periods of silence and isolation become both longer, and more frequent.

The simdroni's body grows longer and thicker as its influence becomes more noticeable. Once the stranger's weight is greater than that of its victim, its effect turns lethal, as the affected individual loses all desire to verbally communicate, then ceases all attempts to physically communicate, as well.

The socially diminishing effect is irreversible, and as the simdroni grows to fill the bulk of the room, a victimized individual loses the ability to recognize, relate to, or interact with other people altogether. If the stranger's victim is not physically moved, they will wither away inside their home, sentenced to a quiet, lonely death.

The simdroni generates in a set of two 0.1% of the time. These pairs of simdroni are inert, and have no effect on people within their environment. These twins display a limited range of motions in comparison to their lone companions; they are more likely to engage in repetitive motions, and seem fixated on one another. As they grow larger, each twin grows weaker and less active, until they both stop moving and fade away.

Though its body is hypothetically able to live for hundreds of years, the simdroni always dies once its victim dies, is killed, or is removed from the home.

The simdroni's behaviour does not change in the moments leading up to death. It instead fades away, turns transparent, then dissipates into a black, gaseous shape. This blur fades within the half-day, leaving behind a stain that resembles a human's shadow more than that of a stranger.