Dusk of Oolacile

Dusk of Oolacile is a character in Dark Souls who sells unique, miscellaneous sorceries. She is voiced by Emma Pierson.

Location

 * Dusk is trapped inside a Golden Crystal Golem located behind the Hydra in Darkroot Basin. After defeating the Hydra, players may need to exit and reload for the golem to spawn.
 * After killing the golem, Dusk will be available as a summon sign that should be on the ground right before entering the lake of the Hydra. Reverse hollowing is not required to summon her.

Plot
Dusk is an entirely optional character; she sells the player miscellaneous spells as a reward for rescuing her. These include interesting things like Fall Control, Cast Light, and Chameleon, so depending on the player's playstyle it may be well worth the trouble of rescuing Dusk.

While it is not necessary to the main storyline to interact with her, it is necessary if the player wishes to experience the events that take place in the Artorias of the Abyss DLC.

Lore
Dusk is from the ancient and long-dead land of Oolacile, which practiced magic alien even to the magic realm of Vinheim. It is unknown how she was trapped in the Golem in the first place, but it is clear that she is in the wrong era.

Strategy
She does not fight back.

Drops
She drops nothing for the matter, so there's really not much point to killing her. The Dark Hand can steal nothing from her.

Trivia
Dusk's voice actor, Emma Pierson, also voices Anastacia of Astora.