Yeerk

The Yeerks are a fictional alien species from the Animorphs series of young adult books by K. A. Applegate. They are parasitic slug-like creatures that seek to conquer other sentient species by infesting their brains and controlling their bodies. A Yeerk enters its host through the ear canal, typically while the host is asleep, and attaches itself to the brain stem. Once attached, the Yeerk assumes complete control, forcing the host to act according to the Yeerk's will. The host's consciousness is not destroyed but rather suppressed, trapped within its own mind, aware of everything but unable to act independently.

Yeerks come in various castes, with the Andalites and other species often serving as primary hosts. Their society is hierarchical, with a ruling class known as the Yeerk Pool, which is a vast underground reservoir where Yeerks gather and receive orders from the Ellimini, the ultimate ruling council. Yeerks are driven by a desire for domination and sustenance, viewing other species as mere tools or resources. They possess advanced technology, including sophisticated starships and weaponry, which they use in their galactic conquest. Their primary goal is to establish a vast empire by enslaving all intelligent life. The Animorphs, a group of human teenagers, fight against the Yeerk invasion of Earth.