From Shadow Era Wiki

Ambush is a card ability in Shadow Era.

Cards with Ambush are able to attack opposing allies or heroes without being retaliated against in battle (e.g. Nightshade can attack a full health Jasmine Rosecult without Jasmine defending back and killing Nightshade). Generally, allies have this ability, although it can also be given to them by other card abilities (e.g. Lance Shadowstalker), and some heroes or weapons are able to have this ability as well. This ability was introduced in Call of the Crystals.