Amnion appears as an androgynous humanoid figure attached to a large mechanical apparatus resembling a spider. The human body is pale and bloated, like a waterlogged corpse, or a woman in the final days of pregnancy. Amnion has three pairs of breasts just above its inflated abdomen. It has no arms and its six spider-like, metallic legs are fused into its ribs. Another feature is a hose-like, black structure that comes out of its mouth and attaches between its legs, like an umbilical cord; however, it convulses and flows as if it has fluid circulating in it. Amnion's face also telescopes forward, all attached to a mechanical frame which fits back together. The spider limbs end in very wicked, claw-like instruments, much like those of the surgical tools seen in the game's opening scene.
Silent Hill: Homecoming
Amnion appears when Alex discovers the ritual chamber where the Order buries their sacrifices for the pact with the founding families of Shepherd's Glen. After recalling accidentally killing Joshua by taking him to the lake and starting an argument over the family ring, Alex learns he was destined to be sacrificed. It was not Joshua's death that ruined the ritual, however. The emotional strain of now lacking both children broke Alex's parents, and so they refused to carry out the sacrificial ritual. After this sudden realization towards the game's end, Alex's surroundings are invaded by the Otherworld, and Amnion descends from the ceiling, initiating the final boss fight.
After defeating Amnion, Alex slices its bloated stomach open, revealing Joshua's corpse, allowing Alex to finally accept what he's done. Taking responsibility for Joshua's death, he begs for his brother's forgiveness and leaves him with both the ring which they fought over while on the boat and the flashlight that Alex had given to Joshua years before.
Amnion has a wide attack radius and can knock Alex down with most attacks. It can take a great deal of damage before falling. It attacks both with wide, sweeping motions and vertically angled assaults with its front legs. When it inhales and extends its face, it sprays an incredulous amount of toxic bile from its mouth that can damage Alex. When he comes within arm's length of the creature, it will sometimes leap straight up in the air and slam down, knocking Alex back if he doesn't evade the assault in time.
After taking significant damage, Amnion will shift its form; no longer depending on its mechanical legs, it now faces forward, standing upright, using the forelegs more like arms. Its attacks in this state are swifter and more brutal, retaining its attack pattern from its previous stage while varying it with the addition of various new attacks. It will slam both of its arms down, which inadvertently causes them to get stuck in the floor, leaving it vulnerable to attacks for a short time. It can also spray its toxic bile up in the air, causing a rain of the deadly liquid. Finally, it can also grab Alex when he's underneath its legs, lifting him up and throwing him down without giving him a chance to break free.
Amnion could be seen as representing Joshua's anger towards Alex for his death, and also his desire to have Alex accept what he'd done. Another view on the creature argues that it could represent Alex's guilt regarding Joshua's death.
Its form is derived mainly from Joshua's death via drowning and his love of insects and spiders, as evidenced by his collection and drawings. It is also representative of the fact that Joshua was "babied" by his parents, and was in fact called a baby by Alex not long before his death; this notion is supported when Alex cuts Amnion open to see Joshua's body in the fetal position, relative to its namesake. It slightly resembles Arachne, a Greco-Roman spider woman who could not acknowledge where her knowledge came from, and may be a representation of Alex refusing to accept his brother's death.
Further symbolism of the theme of birth and Joshua's death include: the name "Amnion" referencing the amniotic sac that surrounds a fetus, the tube of water protruding from her mouth referring to the act of drowning as well as the imagery of an umbilical cord, and the iron lung component used for Amnion's "neck". The body of Amnion's first form is also clearly bloated and gray in tone, appearing very sickly and deprived of oxygen — traits very commonly found in a drowning victim. The "leg" contraptions of Amnion appear to have a rust-like texture. Iron gradually rusts when exposed to water, offering more symbolism towards Joshua's cause of death.
The extending neck is reminiscent of a view camera, which may symbolize Joshua's love for photography. The similarity that Amnion's legs have, and also its way of death (its flipping upside down and legs crumbling up), to a spider symbolizes Joshua's love for spiders.
- The photo found in the playground foreshadows the final boss battle, the spider being Amnion while the toy soldier represents Alex.
- Amnion is one of the only two bosses that was not responsible for the death of their parents, the other being Asphyxia; although, in Amnion's case, it was impossible because Joshua and Alex's parents were both already dead.
- In the opening scene, when Alex is pushed into Room 206, a surgical tool hangs over him that greatly resembles one of the weapons of Amnion, foreshadowing the later boss fight.
- Defeating Amnion in the Xbox 360 version unlocks the achievement "Head Above Water".