Etherea is a realm sustained by a form of magical energy called mana; Garvin Usher says that mana is to Etherea what air is to Ramar, permeating the dimension and enabling life. Etherea intersects with Ramar's dimension at specific fracture lines.
Ethereal creatures are extremely dangerous to Ramar's material beings. Wraiths are among the less dangerous beings known in Etherea. Whether by mischance or on purpose, Ethereal beings sometimes cross to Ramar through the fractures between the two dimensions, but normally their sustaining mana rapidly leaches away in Ramar's atmosphere and suffocates them; they deteriorate faster the further they are from a fissure. However, it is possible to shield an Ethereal creature so that it can survive for a long time despite Ramar's materiality. A sword or staff enchanted as a Wraithslayer is needed to destroy such beings.
It is also possible for Ethereal creatures to crossbreed with creatures in Ramar, producing highly intelligent, powerfully magical hybrids, such as griffins and Fire Wolves, which can easily survive in both worlds.
Wardstones, which cost a pretty penny and are sometimes used by wealthy travelers to invoke the Triune's protection, can be used to block the approach of Ethereal creatures. A circle of wardstones facing outward prevents Ethereans from getting in, while a circle facing inward, such as the broken circle found in the Ridgewood, might be used to hold Ethereans trapped within it.
Mana which leaks from Etherea into Ramar through the fracture lines is the catalyst by which mages manipulate the forces of reality. Mages often feud for possession of fissure sites, wishing to build their sanctums there because magic is easier in areas of greater mana flow, and have lost their lives trying to create fractures to exploit. However, mana's power "works on a mage like wine," causing addictive behavior and making it too easy to lose control, with disastrous results.
The distance between two fractures in Ramar does not necessarily reflect the distance between the same two fractures in Etherea. It is possible to enter Etherea through a fracture, leave through a nearby fracture, and reenter Ramar a great distance from the point of departure. Long ago, someone figured out how mages and possibly non-mages might use fracture-jumping to reduce travel distances. However, fracture-jumping places travelers in danger from Ethereal beings far more powerful than Wraiths; prolonged exposure to Etherea's pure-mana atmosphere can act on mages as a powerful narcotic, overwhelming and scorching them from the inside out; and fractures cannot be opened from within Etherea, making it necessary to rely on a trustworthy mage on the other side to let travelers back into Ramar again. For these reasons, Ethereal travel has been outlawed for centuries, and the knowledge of how to fracture-jump has been lost.