Sister Isis' goggles
You kept these goggles after you defeated Sister Isis, the leader of a Knives of Artemis cell. They're a reminder of the adventure you like to call:
It all began when Arbiter Daos learned of a group of Knives attacking an Arachnos base. They were there looking for information, but beyond that, you didn't know their purpose. You eliminated the Knives and learned that they were seeking the whereabouts of an Arachnos operative they referred to as Moira.
Arbiter Daos knew of no Moira, but a bit of digging revealed that a Fortunata called Mnemosyne had previously gone by that name. She had been a Knife of Artemis before joining Arachnos, and now it seemed that the Knives wanted revenge for her defection. You went to Mnemosyne's base and escorted her out for her own safety, dodging attacks from the Knives as you went.
Then, it was time for revenge. Arbiter Daos knew that an attack against Arachnos would have to be met with sudden, brutal, and unrelenting force. That force was you. You eliminated an entire Knife cell, spreading word of your power, and that of Arachnos.