Invisibility (DMG)

Invisibility is a highly useful tool for both player characters and DMs. Handled well, it can create surprises and unexpected encounters. However, invisibility requires careful judgment on the part of the DM, lest situations occur that could unbalance a scenario or campaign.
First, an invisible creature is invisible to everyone, including itself. This is normally not a great difficulty; most creatures are aware of their own bodies and don't need to see their feet to walk, etc. However, when attempting detailed actions (for example, picking a lock or threading a needle), invisible characters have serious problems, suffering a -3 (or -15%) penalty to their chance of success. This does not apply to spellcasting.
Second, invisible characters are invisible to friend and foe. Unless care is exercised, it is easy for a visible person to blunder into an invisible companion. Imagine a fighter swinging his sword just as he realizes he doesn't know where good old invisible Merin is standing! The problem is even worse with a group of invisible characters—characters crash and tumble (invisibly) into one another, all because nobody can see anybody. It would be like having a roomful of people play pin-the-tail-on-the-donkey all at once!
==Detecting Invisible Creatures==
Invisible creatures and things are not detectable by normal sight or by infravision. They do not create any significant distortion or haze pattern that can be noted. However, invisible creatures aren't completely undetectable. First, things still cling to them. Flour thrown into the air is useful for this purpose, although it can be easily covered, washed off, or brushed away. Second, they do not leave invisible footprints. Again, flour on the floor is a good way to spot the movement of invisible creatures.
The effects of specific environments are more subtle. Fog and smoke do not reveal invisible creatures. Smoke and fog are filled with swirls and eddies, preventing the creature from being detected. Invisible creatures completely submerged in liquids are also concealed; there is no hollow space or "air bubble" to reveal the creature's presence. At the surface, an invisible swimmer may be noticed by the observant as an unusual distortion of the waves.
Invisible creatures are not automatically silent. An invisible fighter in plate mail still clanks and rattles as he moves, a dead giveaway to most creatures. They still have scent, so creatures with keen noses can smell them. Indeed, blind, or nearly blind, creatures are unaffected by invisibility.
A detect magic shows only the presence of something magical without pinpointing it exactly. Thus, it cannot be used as a substitute for a detect invisible spell. Furthermore, while an actual light source may be invisible, the light emanating from it is not. This can reveal the location of an invisible character.
When the DM thinks there is minor but sufficient cause for a creature to detect an invisible character, a saving throw vs. spell should be made (secretly if the DM is checking for a player character). A minor cause might be a strange odor, small noise, an object that disappeared when it shouldn't have, or a strange reaction from another person (who has been pushed, kicked, poked, etc., by the invisible character). Such a saving throw should be allowed for each new event. A wolf would get a save when it detected a strange scent, then shortly after when it heard a stick break, and finally a last chance when the character drew his sword from his scabbard. Furthermore, the acuity of the creature's senses and its general intelligence can increase or decrease the frequency of checks, at the DM's discretion.
If the suspicious creature or character rolls a successful saving throw, he detects some small sign of the invisible foe's presence. He knows its general location, but not its exact position. He can attack it with a -4 penalty on his chance to hit. If the check fails, the creature or character is unaware of the invisible opponent until it does something else that might reveal its presence.
Of course, a revealing action (which could range from an attack to tripping over a pile of pots) immediately negates the need for a saving throw. In such cases, the character has a pretty good idea that something is not right and can take actions to deal with the situation.
Finally, even if an invisible character is suspected, this does not mean the character will be instantly attacked. The result, especially for less intelligent creatures, may only be increased caution. Having scented the intruder, the wolf bristles and growls, protecting its cubs. The rattlesnake will give its warning rattle. Even the orcs may only circle about warily, alert for an ambush.