Vessel Shield
The Vessel Shield adds one point to each of Vigor, Endurance, Adaptability, Intelligence, and Faith. Furthermore, it increases Strength and Dexterity by four points each. The Vessel Shield, together with other equipment that boost your stats, allow you to two-hand various weapons such as the Long Bow, Avelyn, and even the Greatsword. For example, the Vessel Shield and the Strength Ring are enough to allow you to properly two-hand the Greatsword to defeat most strong enemies.
Frozen Eleum Loyce
Collect the Eye of the Priestess. The item allows you to see enemies that were previously invisible or are barely visible as ghostly apparitions. In particular, the Eye allows you to see the boss Aava.
Aava, the King's Pet
Buff the Raw Rapier+10 with Charcoal Pine Resin or use a Rapier that has been infused with a Firedrake Stone. Have at least 14 Adaptability to allow you to roll through Aava's attacks.
Location of Vessel Shield, Dark Souls 2: SotFS
Defeat Aava, enter the Grand Cathedral to light its bonfire, and exhaust the dialogue of Alsanna so she would dispel the blizzard that has been raging throughout Frozen Eleum Loyce. Alsanna would also open up various passages that were previously blocked by ice as well as unfreeze a number of treasure chests. The iron chest at the Grand Cathedral contains the Vessel Shield.