From Shadow Era Wiki

Stealth is a keyword ability in Shadow Era.

Stealth allows any hero or ally with stealth the ability of being untargetable for attacks. This is shown in the game by a fogged out animation. Some cards always have stealth (e.g. Nightshade), others will grant it to a target for a limited time (such as Moonstalker). Some cards, such as Wulven Predator, are able to attack regardless of stealth. This ability was introduced in Call of the Crystals.