Drow chain mail is a special type of armor that exists in many of the AD&D campaign worlds, and also in the Slash'Em variant of Nethack under the name of "Dark elven mithril-coat". This is the primary form of armor used by the Drow, an evil race of dark elves who live underground.
This armor is a perfectly crafted tunic of satiny black metal mesh. It is almost weightless and doesn't restrict the wearer's movement at all. It is much like the magical variety of elven chain mail but with several significant differences. It must also be noted that unlike the ultra rare elven chain, drow chain mail is very common, but only among the the Drow.
Drow chain is made from adamantite that is combined with an unknown metal to create a special alloy. This armor is created on a secret forge, and gains special properties from the natural radiation of the Drow homeland. All Drow chain has the equivalent of a +1 magical bonus, but this can go as high as +5 if the armor spends a long time in the forge. Despite the bonus, Drow armor does not radiate any kind of magical energy.
A suit of Drow chain is like a Vampire in a way. Any exposure to sunlight will remove all of its magical properties, and cause the armor itself to fall apart after a few weeks. Even when kept out of the sunlight, the armor must still return to the Drow homeland at least once every two weeks, or it will be ruined. The Elves of the surface point this out as a failure on the Drows behalf, that they cannot fashion proper armor. While the Drow (assuming you can get one to talk to you), claim that the armor is made that way on purpose to prevent it from being used against them.
The few scholars who have been able to examine both Elven chain mail and Drow chain mail have found them to be remarkably similar. Which gives evidence that perhaps sometime in the past, that the Elven races lived together. They say that the two types of armor were descended from a single type many years ago.