"I had seen more battles, death, and destruction than many seasoned soldiers in our Andalite fleet. I felt my throat tighten and constrict. My hearts ached with a pain I could not describe. I wondered if I were dying. I felt not sadness. I felt pity. For myself. For us all. We were children no longer. And we never would be again."
The Yeerks are a parasitic alien species. In their natural state, they resemble large slugs, and live in lakes of sludge known as Yeerk Pools. The Yeerks have the ability to enter another being's body and take over all brain functions, including basic motor and speech control, essentially "becoming" that person. The Yeerks run their civilization as an Empire, ruled by a Council of Thirteen. The Yeerks were discovered by an Andalite named Seerow, who gave them the means to leave their planet; the Yeerks took advantage of his kindness, and used the Andalite technology to invade and conquer other species, for the sole purpose of expansion.