From the publisher's website:

Small World and Small World Underground just got a lot bigger with this expansion... Pair up in teams and attempt to rule the land together. Despite the growing landscape, you'll find there still isn't enough room to accommodate you all! This double-sided map gives you a new way to play Small World and Small World Underground. Teams of two players combine to compete against others, with each team member playing their own Race/Special Power combo and keeping their own Victory coins. But there is an ingenious twist... it's the point total of the LOWEST team member that decides your score against the other teams! Contains: A 2-sided, 6-player board map featuring Small World on one side, Small World Underground on the other side New rules, places and a relic, all specially designed for team play Additional tokens and markers for 6 players (1 Mountain, 2 Black Mountains, 1 Vengeance marker, 3 Lost Tribes) Additional Victory coins for 6 players