The Berserk was added in Play the World, and replaces the Longbowman. Berserks have extraordinary attack and a mild defensive bonus compared to Longbowmen, and are able to launch amphibious assaults.
Civilopedia Entry
In the pagan era, before Scandinavia was converted to Christianity, the berserkir warriors were looked upon as possessing supernatural powers attributed to the Viking's chief god, Odin and a belief in lycanthropy. These warriors were known to rush into battle without armor, as mad as wolves, and as strong as wild boars. This infamous berserkir frenzy reportedly allowed them to kill men in single blows with great bearded axes and shrug off blows from fire and iron alike. Today it is believed that these frenzies were the result of epileptic attacks.