The astral plane (or luminiferous ether) is a world which exists near our world (the material plane) but separated from it in an arcane and unknowable direction. Normally, beings in our world have no ability to see or influence the astral plane. However at least some creatures in the astral plane are either dimly visible from the material world, or can interact with the material world at will, or both.
Alas, astral creatures, at least those who interact with the material world, are very often malevolent. Or perhaps they perceive material creatures as a horrifying threat. In either case, the result is the same.
Some astral creatures have the ability to move unliving things, but not to directly influence living creatures. For example, they might be able to cause stones to fly at a person. Their strength, if we may call it that, varies widely. Some may do little more than cause a book to fall off a bookshelf, or a door to close. Others have superhuman strength.
Other astral creatures have the ability to influence living things directly. Some will punch a person across the face, or cause scratches to appear. Others will seem to stroke their hands across a person's heart, causing all but the bravest to be overcome with fear.
If a person stands in a position such that they are occupying the same space, as it were, as any astral creature, it has the same effect on the heart, even if the creature cannot cause this effect at will. Astral creatures themselves never seek to cause this effect, and indeed will actively avoid it. Perhaps it has a similar effect on themselves.
There are some such creatures who seem not to perceive material creatures, and so pose no danger unless a person gets in their way.
There does not seem to be a similar variation in the quality of material creatures. If an astral creature can see any material creature, it will be able to see all. If it can stroke the heart of one creature, it will be able to stroke the heart of all. Whereas there are visible and invisible astral creatures, there do not seem to be visible and invisible material creatures. It may be that what we call by the one name of astral creatures, may be several kinds of creature.
The superstitious are known to mutter that astral creatures are unquiet spirits of the dead. There is some evidence that these creatures favour places of death. It is speculated that the presence of death in the material plane may provide life in the astral plane. Perhaps our battlefield or slaughterhouse becomes a pleasant garden or abundant forest in the astral plane. More horribly, perhaps a pair of lovers who stroll through a sun-filled park appear in the astral plane as dim figures in the thick smoke of a crematorium. However, the wise are not taken with stories of spirits, and know that astral creatures are nothing more frightening than eldritch extra-dimensional entities who may wish to destroy all life.
Certain weapons have the virtue of existing in both worlds at the same time, and thus are effective against astral beings. There are also rituals which will render an area safe against such entities. Whether they forbid them from entering, or merely render them invisible and intangible, is unknown. Such rituals are far more difficult, if not impossible, if such entities are already present; just as a bargain not made may with ease never be made, yet once made, what a labour it may be to unmake.
The smoke from wood sprinkled with certain of the Noble Spices is said to reveal some otherwise invisible spirits. It does not appear to harm them.
Astral cats are bred for the fact that they live in the material world and the astral plane, at the same time. This means they can sense, and interact with, creatures of the astral plane. Astral creatures, unused to any threat, will often flee in terror from them (or at least, it is assumed that they feel terror). However astral cats are no more fearsome in combat than any other domestic cat.