Yeerks

The Yeerks are a fictional alien species from the Animorphs series of young adult books by K. A. Applegate. They are slug-like creatures that possess the ability to enter the brains of other living beings, taking over their minds and bodies. This process, known as infestation, effectively turns the host into a slave, erasing their personality and free will. Yeerks reproduce by laying eggs and require a constant supply of new hosts to survive and expand their empire.

Yeerks originate from the planet Eask, which was destroyed by a galactic war. They are a highly militaristic and hierarchical society. Their ultimate goal is galactic domination through the widespread infestation of sentient species. The Yeerk hierarchy is led by the Yeerk Pool, a central hub where new Yeerks mature and are assigned hosts. The Andalites, another alien race featured prominently in the series, are aware of the Yeerk threat and have been fighting against them for centuries. The Animorphs, a group of human teenagers, are given the ability to morph into animals by the Andalites, allowing them to fight the Yeerks on Earth. The Yeerks are a primary antagonist throughout the entire series.