After you defeated the hero Infernal, you kept his axe. It's a powerful weapon, as well as a reminder of the adventure you like to call:
It began with Operative Rutger's mission to prevent Infernal from establishing a new base on Nerva. You interrogated several Longbow agents in the area, and on one of them found a computer pass key. This would enable you to access any files on Longbow's computer network.
You next went to a Longbow base, where you used the computer key to access information about Infernal's new base. You learned not only its location, but also its name: Firebase Valhalla.
You needed some help from Crey, so once again you took to the streets. After defeating a number of Crey agents, you learned that Crey was working on some communication jamming equipment.
You stole the equipment, caring little for the turmoil you caused in a Crey facility.
You went to a Longbow communications hub and installed the jamming devices. When you came for Infernal, he would never have the chance to call for reinforcements.