Laestrygos gave you a Piece of Aegis and said Anusis could combine it with your greatsword to make a stronger weapon. Go to Anusis in Sanctum.
Step 1. Take Laestrygos’s Greatsword, a Piece of Aegis, and a handiwork fee to AnusisA master craftsman who ascended late in his life. He knows about all legendary swords, and can be found in Artisans Hall.. (You need to collect: Laestrygos’s Greatsword)