The Shadow Goblins come from the other side of The Portal! Which Portal will you ask? As you know, the Dark Goblins that you discovered in the main game have a trapping power. When they use it, as you may see on some artworks, the trapped goblin is in effect thrown out through The Portal. For decades now, returning goblins have reported having encountered members of a mysterious and frightening goblin clan … The Shadow Clan!
These goblins are very similar to the Dark Goblins, mainly because of the two powers they have in common: Trapping and Zombie Mutation.
These new goblins will also bring along two new types of gameplay. First of all, some of these goblins will arrive in play already trapped, compensating for a greater power. Secondly, other goblins will introduce a new kind of action during his turn, like [*Trap Zäxg* : Choose 1 FREE card then destroy it.]. In this example, your action will be to trap Zäxg for free in order to destroy a card. This action is free and only requires the trapping of the goblin card having the power.
From the designer's website: http://www.moonstergames.com/2010/10/06/gosu-les-gobelins-de-lombre-the-shadow-goblins/