Secret Weapon
Either something or someone that is hidden and no one knows about and that give's an advantage over the enemies.
Overview

The term 'secret weapon' is used to either describe a weapon that is concealed from another, or a weapon that has not been confirmed by its user. It could also come to describe a person which someone believes will come to their aid, or defense, or help them archive something which the public or individual have no idea about. In the terms of a large scale weapons, it is also a newly developed secret weapon created by a government such as the Atomic Bomb created during the 1940's by the United States of America.
As a theme it has been a frequent subject in popular culture in comics. It has been used to add an element of suspense such as when a hero is either attempting to stop the use of an arch enemy's secret weapon or having an ace in the hole when it comes to a fight.