Inquisitor of Atom

Inquisitor of Atom is a perk in the Fallout 4 add-on Far Harbor.

Obtaining
This perk is given as possible reward for the quest Cleansing the Land.

Effects
Gain increasing amounts of damage depending on your irradiation (maxes out at +100% at >300 rads)

It's also worth noting that bonus damage applies to ballistic damage, energy damage and pure radiation damage only. It doesn't affect explosive damage and radiation poisoning damage. (Works exactly the same way as the Bloody Mess perk)

You can make use of the Robes of Atom's Devoted's exploit to gain a permanent +100% damage without going through the trouble of irradiating yourself to gain the maximum bonus.