Very rarely, you can find "Pieces" to some sort of object scattered around.
Bring them all together and then you can interact with them to start filling up your [Doing something...] bar, it text is "Assembling.", and creates loud sounds during the process of it.
anybody who see's you assembling, will instantly go hostile, no matter their allience to you or their buddies, Once it fills up, you deploy the giant bomb.
What to do once thats done?
get the hell out of there!
The game gives you a minute to evacuate the area once its deployed, once it hits zero, the players screen will flash white [no matter the distance], and anybody around that area will INSTANTLY DIE. oh and it will break any object, even if that object is "indestructable".
The giant bomb could be apart of a characters Big Quest or something.