The Gauna are an eldritch spacefaring shapeshifting race and cosmic horror element in the fictional universe of the Knights of Sidonia manga and anime.
In 2171 AD, a Mass Union Ship was discovered at the Solar System's Outer Edge. On board were trillions of Gauna, and for the next 200+ years, humanity lived in uneasy peace with them. Until 2371 AD, when the Mass Union Ship approached Earth and 48 Gauna assaulted the planet. Killing and absorbing humans, as well as other life on the planet. Eventually destroying the Earth, as it split into two hemispheres with kilometers-thick placental arms emerging from the core.
During the catacalysmic invasion of Earth, the humanity raced to build seed ships to take what they could of the human race away from the Solar System. They realized they could not defeat the Gauna, and therefore focused on assuring the continuation of the human species. Hundreds of seed ships were sent from Earth.
Gauna is a lifeform that consists of a True Body and a Placenta. Which has shapeshifting abilities and numerous forms. Gauna cannot be killed or destroyed as it continuously regenerates, unless you pierce the True Body with the spear-like Kabizashi (rare weapons made of Placenta, a Kabi (material from an artifact), and a handle).
The exterior covering of the Gauna is the Placenta, and its core is the True Body. The True Body looks somewhat like a comma with an orb-like "brain" connected to the "spine", a spindly tall form. Which if damaged or "destroyed" can completely regenerate given enough time. The Placenta is highly malleable, can extend great distances, is able to move at incredible speeds, and can morph into various forms. Placenta can create various materials and complex machinery, including biological Heigus Particle Cannons, human organs and eyes, Graviton Radiation Emitters, and a membrane that deflects attacks from Hegius cannons.