According to Lagia, you can strengthen Silion’s Sword with Paruam’s Iris. Kill Paruam, remove his iris, and pay Lagia a visit.
Step 1. Procure Paruam’s IrisA gooey bit of Paruam Sealguard’s eye. Gee, won’t the other Daevas be impressed? and bring it, along with your sword, to LagiaAn Elyos Sentinel who is eager to get into battle.. (You need to collect: Silion’s Sword)