A trinket from before her amnesia that somehow fell out of Aria Wren's possession and found it's way into the hands of Felix Rex. He has since returned it to her (in exchange for the instructions to use the Shield of Force).
The locket is finely made, depicting a bird with the name “Wren” engraved on the back. Inside are pictures that appear to be a younger Aria Wren and Sabine Wren.