The Youluyessess are a race of radiation-energy beings that live in mechanical, door-like bodies. They can posses dead things.
They tried to destroy the humans of the frontier world Raxine. They used President Harmon and converted all the skeletal zombies of Gaarrak to attack Stronghaven.
The skeletal army of death captures Matt, and a skeletal Harmon sends a message to Michael, telling him to bring him the nuclear core of Stronghaven, when he does, he brings a suit protecting him from the Youluyessess energy. Michael finds and releases them and they manifest in Hamilton. After rescuing Matt, Michael destroys the nuclear core and the Youluyessess.