From TheWarWiki

Creation of the Black Arks

During their warring with the High Elves, the Dark Elves' mightiest sorcerers cast powerful spells on the strongest of their fortresses, converting them into massive floating fortresses in order to avoid their destruction at the hands of the titanic tidal waves. In WAR, the Black Arks have landed on Ulthuan, specifically on the Blighted Isle, and both High Elven Players and Dark Elves start their adventure near a Black Ark.