Ergs are a species of semi-sentient extraterrestrial creature. They can generate and manipulate forcefields, and are used in the propulsion and atmosphere-containment systems of bioengineered structures such as treeships and startrees. They were first discovered on asteroids around Aldebaran nearly three centuries before the time of the Final Shrike Pilgrimage. Templars are capable of communicating with them, although how this is done is kept a secret.
Ergs are serpentine creatures, described as being about as big as a cat's spine, and consisting primarily of a piezoelectric nervous system sheathed in silicon.
SPOILER ALERT! The section(s) below are MAJOR SPOILERS.
Multiple ergs were used in the Sequoia Sempervirens to generate an extremely powerful forcefield around the treeship to protect it from incessant Pax attacks as the ship and its passengers were freecasted by Aenea as they dropped off her followers on dozens of worlds to spread her message and DNA sacrament in preparation for the Aenean Shared Moment.