This vendor sells various 'Ancient magicks of Oolacile' plus a catalyst.
After beating the Hydra, there is a small cave to the rear of the lake. Quit your game and load it back to see a golden golem (like the ice/crystal golems you already fought before the hydra). Nothing really special about the attacks. After defeating it, Dusk will appear and ask if you will pass on the knowledge of the Oolacile magicks. Say yes and she will put down a sign at the edge of the water to summon her when you want to barter with her. She can be summoned whether the player is hollow or human.
|Oolacile Ivory Catalyst
||Allows to cast magic
||Turns body nearly invisible
||Cast Light in surrounding area
||Repair equipped weapons and armor
||Transform into something inconspicuous/item in the environment
||Turns right weapon invisible