When a Phazon Leviathan struck the planet of Tallon IV, it spread highly mutagenic Phazon all across the land. The Chozo colonists were unable to stop the contamination from spreading and many of them were corrupted and killed by the malignant substance.
Those Chozo destroyed by the Phazon would become doomed to a death without end as even their very souls were tainted by the corruption. When their bodies died, the spirits of the deceased Chozo remained bound to the mortal plane and had become aggressive and violent, lashing out at their living brethren and any other creatures that crossed their path.
When investigating the Chozo Ruins on Tallon IV, Samus Aran encountered a number of these Chozo Ghosts. Samus was unable to see them without an X-Ray visor and could only dispatch the entities with her Power Beam as no other weapons would affect them. After Samus destroyed Metroid Prime and all Phazon on Tallon IV, the Chozo Ghosts were believed to have been freed from the mortal realm and allowed to pass into the hereafter.
Chozo Ghosts are invisible to standard vision and can only be seen through an X-Ray visor. They are capable of phasing in and out of the material plane, making it difficult to get a target lock. They attack by casting blasts of ethereal energy which causes Samus' suit to malfunction, disrupting her visor and HUD. Chozo Ghosts are also immune to all weapons except for the Power Beam.