The Fire Opal is a powerful precious stone that is the source of the Soul Eaters' power and one of the things driving the plot of Michelle Paver's Chronicles of Ancient Darkness series that is set 6000 years ago in prehistoric Europe during the Stone Age.
Appearance and Traits
The Fire Opal is a beautiful stone that burns with fiery and mesmerizing red light, but it's cold to the touch (Wolf describes it as the "Bright Beast That Bites Cold" in contrast to the "Bright Beast That Bites Hot", which means fire).
The Fire Opal is said to be light from the red eye of the Great Auroch, the strongest demon to have existed and a celestial body of great evil. That's why it draws demons towards itself. Whoever holds it can control demons as long as it does not touch ground or stone.
Apparently the Fire Opal also grants the wielder control and protection over fire, as shown with in Soul Eater and Oath Breaker. And it seems to hold at bay guardians of a portal to the Netherworld that's situated near the Walker's valley, as shown in Wolf Brother with the way they reacted to Torak's father's knife which held one of the Fire Opal fragments at the time.
There is only one way to destroy the power of the Fire Opal. It must be buried under ground or stone and a living creature must be sacrificed at the same time. Only then the Opal's red light fades away.
Role in the Plot
It is unknown where the Soul Eaters originally received the original Fire Opal, but they held the Forest within their grip of terror with it. But when they attempted to release demons trapped inside a hill that would later be known as the Burnt Hill, Torak's father shattered the Opal into 3 pieces. But each piece remained as powerful as the original piece.
1st Piece Presented
This piece presented in Soul Eater is about the size of a duck's egg. It had ended up in Eostra's possession. She attempts with Thiazzi, Seshru and Nef to release demons trapped in a Far North mountain called Eye of the Viper and rule the Forest with the shard. But as they release some of the demons, Renn steals the Opal and flees with Torak and Wolf.
Eventually Renn finds a crevasse and intends to jump there with the Opal before the Soul Eaters arrive with Torak. Suddenly Nef takes the shard from Renn and does the sacrifice herself in order to repay her debt to Torak's father. The demons follow her to the crevasse as it closes itself.
2nd Piece Presented
This piece presented in Outcast is about the size of a redbreast's egg. Torak's father took it, but was unable to destroy it, so he hid it in a hole he had carved into his knife's hilt. He passes the knife to his son when he dies. Torak remains unaware of the shard being inside the knife, until his raven friend Rip figures it out.
It is eventually destroyed when Torak smashes it with a rock right in front of Seshru. As the Viper Mage reaches for the shards, Bale shoots her with Renn's bow. Renn ties the shards inside Seshru's fist, letting their power to die with her.
3rd Piece Presented
This piece presented in Oath Breaker and Ghost Hunter is about the size of a blackthorn's berry. It had ended up in Tenris' possession. He used it to trap an elemental (the strongest type of demons) inside a bear's body in order to create a weapon to destroy Torak's father (Tenris' brother). The demon bear eventually does its purpose before it's destroyed. Tenris also used the shard to create two tokoroths to do his bidding.
After Tenris' death in Spirit Walker, the shard remains hidden under the altar of the sacred Crag in the Seal Island until Thiazzi finds it there in the beginning of Oath Breaker. He then goes to the Deep Forest to use it for his own plans to seize control over the Forest. At the end of the book, Eostra's eagle owl steals the shard from Thiazzi before his death and flies off to deliver it to its mistress somewhere in the eastern High Mountains.
The last Fire Opal shard remains in Eostra's possession for most of the plot of Ghost Hunter. She uses it to create tokoroths and corrupted dogs to do her bidding. During the final fight between Torak and the Eagle Owl Mage inside the Mountain of Ghosts, the Walker (formely known as Narrander) summons the Hidden People of the Mountain to create a large chasm. Eostra falls into it with the last Fire Opal shard, ending both the legacy of the Soul Eaters and the Fire Opal once and for all.