The Mark of Eragon

Eragon with patients {subjects} and years of study developed a poison to corrupt a phoenix turning the blood into a serum that creates an undead creature bonded to Eragon threw the special tattoo applied to the dead base of the spine animating the corpse. This carries a component of brain matter of a beholder bonding telepathically to the now controlled undead. There is a 1 in 10 chance the undead can become self aware. Once this happens the bond becomes permanent. A rare side effect Eragon experienced with one of his first thousand attempts with a human fighter Felix Of the Citadels. One of a almost nameless bodies found among the many battles of that time.