Nocturnal Creatures adds a new set of creature cards (Banshees) to the game, and a new mechanic for Wisps.

Wisps
During set up, a Wisp token is placed face down every 5 points on the scoring track, up to space 50 (there are 11 wisp tiles, so one will not be used). Then, all of the tokens are turned over, and the 2-3 "Wisp" tokens are left on their space on the score track. Whenever a player reaches or passes over that space on the track, they may choose from one of the face up Wisp tiles, which gives them an immediate bonus.

Banshees
The expansion comes with 16 banshee cards, 1 in each color for each of the 4 reward types.

Steal: If possible, all players must give a gem of the corresponding color to the player.

Resurrect: Search the discard pile and take a card of your choice (placing in in your discard pile as normal).

Wisped: Take one of the available Wisp tiles and receive the reward immediately.
Card Switch: Destroy the played Banshee card and take a card from the display directly into your hand. Since the Banshee card is destroyed, the other card is placed in your discard pile regardless of number.