Vilirij were mostly active during the night, flying around in search for victims. They also hid over doorways or on ceilings and dropped onto victims who passed underneath. When a victim was found, the vilirij would grasp onto the victim's skin, usually an unseen area of the body, and inject a venom that anesthetized the area of contact and began to affect the victim's mind. This made it extremely difficult for the victim to take notice of the parasite. If viewed by others on an area of exposed skin, the vilirij would simply appear as a patch of discolored skin.
Vilirij fed off the blood of their victims, taking a small amount each day while attached. However, they consumed more blood than their host could produce during a day, so death for the host was a possibility if the vilirij was attached for too long.