Source: The City Outside of Time, pg(s). 82
Chronogeists resemble angry incorporeal versions of their mortal forms clad with the clothing they wore in life and permeated with unhealthy orange or yellow lights. They flash brightly when harnessing the power of unstable time.
After Xin-Edasseril was locked from space and time in a temporal loop that caused the residents to repeatedly relive the week before Earthfall, the souls of the dead could not leave the demiplane for the Boneyard and festered within Crystilan, becoming chronogeists after a few centuries. All memories of the dead were wiped clean from the minds of the survivors after each weekly reset but sometimes appeared in half-remembered dreams, leading some of Xin-Edasseril's scholars to theorise about their true natures (only to forget everything after a reset). When a chronogeist is destroyed, it is absorbed into and weakens the shell around Crystilan.
When a chronogeist is created, it loses most of its memories and haunts the area of its death, confused and enraged about how they are stuck on the Material Plane. In a futile attempt to understand themselves, chronogeists become aggressively territorial and attack any who approach with temporal dangers, while desperately trying to fix the snag in time that led to their undeath.
Chronogeists are barely aware of how they have become near-mindless, evil, territorial undead, except for a few who held strong convictions in life and kept important memories after death. Some of them even continue to fulfil their vows in life, even if they have forgotten why or how and usually in tragic and ironic ways.
Chronogeists are usually solitary due to their strong tie to the place where they died. Where temporal anomalies are common, however, many chronogeists can arise at the same time. These chronogeists often move together but barely understand why, and behave similarly to solitary chronogeists.