A Hallow is a source of Mana that is trapped in the Fallen World. Hallows frequently possess a strong enough flow of magic that even Sleepers can sense the power of the areas. They often have a strong allure for both Awakened and Sleeper, although the latter don't really understand why. More importantly (from a mage's perspective) Sleepers are more likely to Awaken in these sacred places. Not all spots that evoke powerful sensations are currently Hallows, although many are, and many more have been in the past and have the potential to be again if they are properly reopened.
Many strong Hallows tend to be situated in high places, such as on mountaintops or the tips of towers. Other Hallows can occur anywhere, even in the dark between two clefts, or down a deep pit. Hallows also often appear at the intersections of ley lines, and possess the resonance of the associated ley lines.
Hallows produce mana, which can be accessed by Mages who perform oblations within them. These vary from mage to mage, but always include some element of meditation. If no mana is consumed in a Hallow, it builds up, eventually transforming into physical Tass which can be consumed by mages, or taken away and stored for later use. Tass takes the form of whatever is most appropriate in a Hallow. In caves it is often water, mushrooms or moss, in parking lots it can take the form of ticket stubs, empty beer bottles or cigarette butts.