https://www.youtube.com/watch?v=PGF0MYORADE
You find this power in the form of a gun in a mad science lab. Its not the goal of the mission, you will have to find it by poking around.

Just like the video, its an area of effect dealing with entire mobs. It has limited charges, and is just a fun thing you can use or sell.

for that matter you can toss in any number of temp powers that are useful or just for fun. All you need to do is spend a little extra time poking around instead of just killing the mobs and hitting the next mission. It would be a reward for investigating a crime scene.