Roshamuul is the presumed name of a new island, to be implemented in the Winter Update 2013, located east of Darashia. It is inhabited by unholy creatures and mainly targeted at high level players.
The island is divided in two parts by a river, with only one old bridge. On one of the sides seem to be a ruined city and a large wall.
The name Roshamuul comes from the file names found in screenshots of the first teaser. However, in the second teaser screenshot, the name Roschamuul is used instead. It's unknown which one is correct.
For images, take a look here.