http://www.shadowera.com/cards/preview/ll101.jpg
Dark Infiltrator - 2cc Wulven Attachment
Attach to your hero. While your hero has ambush, allies cost you 1 less resource to summon and friendly allies have ambush.
Clarification: Weapons with "Ambush" written on them (e.g. Boooomerang) are using a short-hand form of saying "Your hero has Ambush", since it is only ever heroes and allies that attack (not weapons).