The moredhel - or the Brotherhood of Dark Path, as humans call them - are the Midkemian dark elves.
What comes to appearance, they are not very different from the elves of the south (eledhel), save for black hair and somewhat darker skin and features. The moredhel mostly live in the Northlands. Most humans in the Kingdom (particulaly those in mostly human-populated areas) have problems telling the two kinds of elves apart - but people near the mountains quite probably don't have this problem.
Moredhel are quite warlike and generally far more blood-thirstier than the eledhel. They can attack human settlements and raid places. They sometimes partake in more massive war, as happened in The Great Uprising: The most well-known of moredhel was probably Murmandamus, a charismatic and powerful leader who led the Northlands creatures south to the Kingdom, to reach the town of Sethanon where the Lifestone was hidden (detailed in the book Darkness at Sethanon). Other memorable moredhel characters include Gorath (in BaK, he's described as half-moredhel, but I think he was pureblood moredhel in KtB?), a moredhel general who fought in Sethanon and leaped to the Kingdom ranks in the beginning of Betrayal at Krondor... and Delekhan, the big bad guy from the BaK.
See also: eledhel, glamredhel, elves