Habakkuk is a giant iceberg combined with sawdust used as a dragon-carrier during the Kuusaman-Lagoan invasion of Jelgava. In order to keep the ice from melting in the warmer Garelian Ocean, a team of mages had to periodically cast wards and other spells over it.
Originally, "Habakkuk" was the name of an Old Testament prophet. In the context of the Darkness Series, it is a name taken from the language of the People of the Ice, who are represented as speaking Biblical Hebrew. It is likely derived from OTL's Project Habakkuk, a British proposal to create ships from ice and sawdust.