SCP-538, or "Shadow Spiders", is an upcoming SCP in SCP: Containment Breach Unity Edition that will be added in an as of yet unannounced version.
SCP-538 appears to be animate shadows of an unknown species of spiders. SCP-538 appears to "feed" off of the shadows of other living objects, and will move to the nearest shadow cast by a living organism. To "feed", SCP-538 does no more than attach itself to the shadow of its host in such a manner that its own shadow is not obscured. Through this manner, a single specimen of SCP-538 can grow up to approximately 15 square centimeters in size. Feeding after this point appears to simply maintain this size; the whole process has so far proven to be harmless to the host. While a specimen can attach itself to an inanimate object to feed, it will slowly atrophy and decrease in size over time. Only when connected to the shadow of a living organism can SCP-538 thrive.
SCP-538 has shown itself capable of going short distances through open, well lit areas, such as to reach a nearby host or to escape a source of agitation. However, it will rapidly decrease in size at a rate of nearly 2 square centimeters per second for the length of time it is not attached to a shadow. Should a specimen be stranded out in the open long enough, it will eventually decrease to nothing, at which point it can be considered deceased. Top land speed has been observed at approximately one meter per second when at maximum size. SCP-538 has shown itself to be capable of slipping through cracks greater than 3 millimeters in height; spaces less than this distance appear impassible.
While generally benign, SCP-538 can and will attack its host if frightened. Frightening SCP-538 generally involves a rapid movement by its host, at which point it will "bite" the organism's shadow before attempting to flee. Bite must occur on bare skin to cause effects; clothing material consisting of cotton or anything sturdier will provide sufficient protection.
SCP-538 is not currently in-game.