Klauth, also officially known as the Dragon of the North and nicknamed by the adventurers the "Old Snarl", is a gigantic red wyrm who lives in a vale (known as the Klauthen Vale) somewhere along the north Sword Coast.
Klauth is cruel and evil, but also known for his great knowledge of the Sword Coast area, history and people who pass through. Klauth spends considerable time watching the area, either by lurking in any place you can hide a gigantic dragon, or by using magical devices or spells.
Klauth is whimsical - sometimes he helps people he knows can't harm him, but often just attacks them. He has been able to stomp entire armies without too much effort. He uses his own bulk and many of his spells and magic items to attack. Many have seeked to destroy him, but for obvious reasons and some plot requirements, none has been able to do so.
Most other dragons are extremely afraid of Klauth. One of Klauth's nastiest habits is attacking other dragons in the area to eliminate competition, so very few dragons are foolish enough to settle on Klauth's domain. He even hunts and eats hatchlings and devours unhatched eggs. He also uses some kind of magical process to use the red dragon eggs to increase his own powers and size.
In the official campaign of the CRPG Neverwinter Nights, Klauth also made an appearance as a guardian of one of the Words of Power. In my opinion (and undoubtedly in opinion of many purists), Bioware made a great blunder in this case. First of all, Klauth was a normal-sized red dragon, and secondly, the players needed to kill him without even chance of letting him surrender! (He's a big nemesis, not your random monster! This is just about as sacrilegious as killing Lex Luthor and all of the Beagle Boys!) Not to even mention that hundreds of insane or courageous (or both) people have tried to kill Klauth in vain... I have heard there's a way for evil characters to get the Word of Power without killing Klauth, but there's no way to do that with good characters.
Like most of the Forgotten Realms, Klauth was created by Ed Greenwood, and is apparently detailed further in Dragon Magazine #244 (February 1998), of which I have no copy...
Forgotten Realms Campaign Setting, 3rd Edition
Neverwinter Nights computer game Official Campaign