The Dominion Phials contained the Dominion Virus, weaponized by the Styx to exterminate the Topsoilers, and its vaccine. They were given to the Rebecca Twins by their grandfather, the Old Styx, after testing on the virus in the Bunker was completed. The amount of virus contained in the Phial was enough to start a worldwide pandemic. After the Bunker was destroyed by Drake, and Sarah Jerome cast the Rebeccas into the Pore, recovery of the virus became a priority for the Styx. When they finally recovered the virus, it was useless to them, as Elliott had ingested the vaccine.
The Dominion Phials consisted of two small glass phials, each capped by a different color stopper. The black-topped one contained the virus, while the phial with the white stopper contained the vaccine for the virus.
The Dominion virus, kept alive in Plague Snails, was recovered from the Eternal City by the Styx Division and the scientists. However, the Styx needed to make it more virulent in order to be able to destroy Topsoil society, and also develop a vaccine for it. The Scientists in the Laboratories succeeded on both counts, making it air and water borne, developing a vaccine to it, and making a non-lethal variant of it to test out the delivery mechanism, which the Topsoilers called Ultra Bug.
When the Rebeccas arrived in the Deeps, the Old Styx, who had arrived before them, gave each Styx girl a Dominion phial and a phial of Ultra Bug. One of the twins, upon receiving her phials, told Sarah Jerome about the plot, although Sarah thought little of it at the time. Later, when the twins had Will Burrows cornered at the Pore, they taunted him with the phials, revealing the Styx’s plans to vanquish the Topsoilers and seize the surface. Unbeknownst to them Styx twins, however, they were being watched by Sarah Jerome and Drake, and, after Will and his friends had been blown into the Pore by the Division’s artillery, Sarah, mortally wounded, ran past the Limiter defenses and knocked the Rebecca Twins, who had the phials on them, into the Pore in her last moments of life. Drake, watching, realized that the only source left of the Dominion virus left was the Bunker, which he promptly destroyed, meaning that the only source left of the virus was deep inside the Earth, at the bottom of the Pore.
“Anything? They’ve obviously perished. Concentrate all your efforts on finding the phials! We need those phials!”
After landing on a fungal ledge jutting out of the side of the Pore, the Rebecca Twins took stock of their situation: two Styx girls, a pair of Limiters (who had been ordered to jump into the Pore to atone for their failure to protect the twins), the Dominion Phials, assorted scythes, and the fake phials filled with Ultra Bug and its vaccine. The twins worked out a plan of action: one of the twins and a Limiter would work independently to find a route to the surface, while her sister would insinuate herself into Will’s group and the other Limiter would track them, in case they found a route out of the subterranean maze. The Styx girl who infiltrated Will’s group gave him the Ultra Bug phials to earn his trust, while her sister kept the Dominion phials.
"What if I were to give you this? What if I were to give you Dominion and the vaccine, as a gesture of good faith? Here, take them. They’re the only specimens we had, and…and now they’re yours.”
“How do I know if it’s really Dominion?”
“Because it is.”
“But why would you and your sister have these in the first place. Why you?”
-Rebecca One and Will
When the two groups confronted each other at the Russian submarine, the Styx gave Will a deal, Dr. Burrows (who they’d found wandering the depths earlier) for Martha and the Ultra Bug phials, still keeping up the pretense that the phials Will had were genuine. However, the deal was brought to an early conclusion when Elliott set off an explosion. The Styx (minus one Limiter who’d been devoured by a Bright) took shelter in the submarine, Chester, Martha, and Elliott sought refuge in the main passage, and Will and his adopted father ran into another tunnel. The submarine, containing the Styx and the Dominion phials, was dislodged by the explosions and continued its journey to the center of the Earth. Will and his father found their way Topsoil, where they reunited with Drake, who used the phials to try to lure out the Old Styx.
The surviving Limiter and the Rebeccas made their way to the inner world, where the lack of enemies made them complacent. One day, camped at a pool in the jungle, the Styx girls went swimming, removing their jackets and leaving the Dominion phials unattended. Meanwhile Will and Elliott had been dispatched to the inner world by Drake, and Doctor Burrows, driven to uncover the secrets of the subterranean parts of Earth, accompanied them. That day, Will and Elliott, along with the Hunter Bartleby, went fishing in a river near the Rebeccas’ location. Bartleby detected the Styx’s scent, and Elliott tracked the Rebeccas to their current location. Will stole the phials, but was sighted by Rebecca One, whom he promptly shot with his Sten gun. Elliott sniped the Limiter, and the area went up in flames from the C-4 charges Elliott had set, presumably with the Rebeccas inside. Will then buried the Phials at a spring near his group's campsite.
Rebecca Two, carrying her wounded sister, had jumped into the pool, discovering that it was the end of an abandoned mineshaft. After tending to her sister’s wounds, Rebecca Two followed the mineshaft to the city of New Germania, where she linked up with a company of Limiters sent to rescue the Styx girls and recover the virus.
I want to cut and bleed the little pig, but slowly…so slowly…over days…no…weeks. And he took the Dominion virus from us. He’s got to pay [for that]."
“We’ll get the Dominion back. Now can you just shut up, please. You need to save your strength. I’m going to put patches on your wounds, then bind them really tight.”
-Rebecca One and Rebecca TwoThe Styx intimidated the city’s inhabitants into helping them recover the virus, and, after Rebecca One had recovered from her gunshot wound, set off to find the people who’d stolen their virus. Correctly reasoning that Dr. Burrows would be drawn to the pyramids, an, and that the radio signal emanating from one of the pyramids was one of Drake’s devices, the Styx and their new allies quickly found the group and took Will and Dr. Burrows hostage. Elliott, who’d been in the forest, escaped the initial Styx sweep and reached the spring where the phials were stored. There, she drank the vaccine, and broke its phial to look like she’d accidentally destroyed it in her rush to retrieve it. Elliott put the virus in a sock around Bartleby’s neck. She then climbed a tree to observe the situation at the pyramid. The Rebeccas offered her a deal: if Elliott didn’t give them the virus in an hour: they’d kill their hostages, and while they waited Tom Cox would torture Will, removing one of his fingers every ten minutes.
"Bring us the Dominion virus and all three of you will be spared. You’ve got five minutes to give us a sign that you agree to this… all you have to do is to fire your weapon twice. Then we want the Dominion virus here at the pyramid within the next hour. It’s that simple.”
-Rebecca TwoElliott couldn’t watch this and killed Cox. In retaliation, Rebecca Two killed Dr. Burrows. The deal that Elliott finally worked out with the Rebeccas was that she and Will would be evacuated from the scene in a helicopter, and the Styx would get their virus after they were in the air. As the helicopter lifted off, Elliott called Bartleby and, after he was in the helicopter, took the virus off of his neck and tossed it to the twin Styx girls.
Will and Elliott made it Topsoil, where Drake took samples of Elliott’s blood and sent them to three Topsoil hospitals: St. Edmund’s, St. Thomas’, and the London. Meanwhile, when the Rebecca Twins returned to the Colony via the system of hot-air balloons the Limiters set up in the Pore, they had brought the virus phial with them. The Styx twins voiced their suspicions about the vaccine phial’s fate to their grandfather when they presented the virus to him.
“There’s a tinsy winsy bit of bad news, but we didn’t want to fall into that ‘good news, bad news’ routine.”
“No. The vaccine phial was supposedly broken and its contents lost. But when we thought it though afterwards, we realized that Will or Elliott or both of them might have glugged it down – most likely Elliott, as she was unaccounted for when we first arrived at the scene.”
“We’re not entirely sure about this, though. But if the half-breed or that idiot do have it and they somehow manage to get it into the right hands, say to Drake, th—?
-Rebecca One and Rebecca Two
Their suspicions were borne out when the Styx instigated the hospitals’ vaccine banks, and the Styx, through their puppet Prime Minister, had the infection response units at those hospitals closed, snatching back the vaccine samples. Later, Drake gave his friend Charlie more samples of Elliott’s blood, as well as samples of other viruses he’d stolen from the Laboratories. By this time, the Styx no longer needed Dominion, as they were using the newly-acquired New Germanian Army to take over Britian by force. Also, it was revealed the Styx were entering a new Phase. This meant that it made less sense to kill humans with a virus when incubating Warrior Larvae in a human did the same thing, as well as producing thirty Styx soldiers.