Longships are a type of boat usually used by kvaldir raiders. The kvaldir can be seen using them to attack the tuskarr city of Kaskala.
The Naglfar, a fearsome ship, is used to deliver cursed and damned vrykul souls to Helheim and the clutches of Helya. Several other longships are also anchored in Helheim.
In the RPG
This section contains information from the Warcraft RPG which is considered non-canon.
A longship is a 75-foot-long ship with forty oars and a total crew of 50. It has a single mast and a square sail. It can carry fifty tons of cargo or 120 soldiers. A longship can make sea voyages. It moves about 3 miles per hour when being rowed or under sail.