The Shimmer

The Shimmer from the Southern Reach Trilogy and the 2018 film "Annihilation" is an amorphous being of unknown origin that crash landed on Earth and began to terraform the surrounding area and create what is known as Area X. He is the main antagonist of both versions.

Physical Appearance
It is not known what the Shimmer looks like, but in the film, his appearance looks similar to the 3D mandelbulb. Upon copying Lena (the Biologist) when she sheds a single bloody tear during her encounter with him, the Shimmer adopts a humanoid form that has silver skin.

History
Landing in the middle of a lighthouse off the coast of Florida, the Shimmer began to create a constantly expanding bubble-like barrier where everything that entered it began to (in a metaphorical sense) become refracted like light in bizarre ways such as the DNA of various organisms inside beginning to alter. The area inside said barrier was titled "Area X" and as it began to expand overtime, civilisation began to deteriorate with vegetation and fungi growing at an exponential rate and turning what use to be a small quite town into an alien forest of monstrous creatures of what use to be normal animals. As Area X was studied by a secret organisation known as the Southern Reach, various expeditions were sent into Area X with all of them never returning except emotionally empty duplicates of the expedition members who mysteriously return to the homes of who they appear to be without any trace of how they got there.

It is unknown what intentions the Shimmer has for creating Area X or the empty duplicates (if any at all). Based on the Shimmer's encounter with the Biologist of the twelve documented expedition were the Shimmer begins to change using the DNA from her blood to take a humanoid appearance and begins to mimic her, as if it was studying her movements and actions. Over time the Shimmer began to take on the appearance of the Biologist as their encounter progressed (which inherently explained the doppelgängers). In the 2018 film "Annihilation" which was based on the first book in the Southern Reach Trilogy, the Shimmer was killed by the Biologist by when she tricking him into killing himself with a flash bomb which led to Area X reverting back to its previous state. However it is implied that the Shimmer survived through the doppelgängers it has created.