The Lost Cities: Sixth Expedition promo pack contains a set of grey cards that are added to the base game as an additional expedition. The sixth expedition has the same rules as the other five.
9 Expedition cards with values 2-10
3 Wager cards
Sixth discard pile card (back of instruction card)
With the exception of the changes described here, the rules of the base game remain the same.
Place this card with the reverse side facing up next to the game board. This is the discard space for a new, sixth expedition. Shuffle the new cards into the deck and play as usual, now with six expeditions.
For more information, visit the BGG listing.