Item #: SCP-173
Object class: Euclid
Special Containment Procedures: Item SCP-173 is to be kept in a locked container at all times. When personnel must enter SCP-173's container, no fewer than 3 may enter at any time and the door is to be relocked behind them. At all times, two persons must maintain direct eye contact with SCP-173 until all personnel have vacated and relocked the container.
SCP–173 is the very first item ever created for the SCP Foundation, a fictional organization of top–secret scientists who collect and study bizarre and freakish specimens of supernatural power and hide them from the world, which since this monster’s creation has evolved into a massive fictional database featuring hundreds of specimens and an established mythology. SCP–173 is a strange living statue with bizarre markings that kills all living things on sight, moving toward its victims at a speed of several feet in one blink of an eye and snapping their necks with irresistible strength and lethal precision. In addition, it is completely invincible. It also periodically produces feces and blood onto the ground of its containment cell, which must be cleaned up from time to time (see below).
Fortunately, SCP–173 has one key weakness: it is frozen in place as long as someone is looking at it through eye contact, similarly to the Weeping Angels of Doctor Who lore. Thus, a victim has a chance of escaping from this abomination by keeping their eyes focused on it; however people need to blink frequently and often do so subconsciously, and with each blink SCP–173 makes a large stride toward a person, and can move across the length of a large room in just a few blinks. Thus, one’s best bet when trying to escape from 173 is to walk backwards away from it while keeping your eyes steady. In theory, one could stop SCP-173 by winking one eye at a time (easier done by manually closing one eyelid with a finger and then the other once the first one has been blinked), this is actually much harder to do in practice, as the human mind is trained to blink both eyes at once. Another method would be to not completely blink, but half-closed to decrease tiring the eyes.
Due to its nature, whenever the SCP Foundation sends test subjects to check on 173 they send them in groups of at least 3, so that at least one of them is making eye contact with it at all times. Unfortunately, the subjects’ foolish and defiant behavior can often lead to death in these situations, as shown in this video: 
BITCHIN' NEW MODEL SCP–173 as it appears in SCP: Containment Breach. Tremorfan94Added by Tremorfan94
SCP–173 is classified by the organization as a “Euclid”, meaning it is unpredictable and more difficult to contain than a "Safe" classed SCP. The reason that it is not a “Keter”, the most dangerous of SCP's which are often deemed large–scale threats to society and require strict procedures to contain, SCP-173 is overall very easy to contain, and cannot present a huge threat to large amounts of people. Nonetheless, 173 remains one of the SCP Foundation’s most dangerous monsters to any lone individual, and once even managed to incapacitate SCP–682 in a one–on–one encounter.
SCP: Containment Breach
SCP–173 plays as the main antagonist in the indie-developed game SCP: Containment Breach. It moves when the player is not directly observing it or if the player has blinked. It moves incredibly fast, as already stated, and will snap the player's neck if it gains the opportunity. It travels through the vents and chases the player down throughout the majority of the game.
Inside of 173's head. Tremorfan94Added by Tremorfan94 In one Tale, entitled "crunch", the thoughts of SCP–173 are translated and explained that SCP–173 kills people because when they look at it, it becomes "prisoner". It also hints that not only is SCP–173 sentient, but it is also an organism except when it is being observed. ("Free when they not look. Prisoner when they look back. I hate.") It seems to have a desire of killing and finds purpose in snapping people's necks, suitably because of the crunch sound.