A few years before the Royal sisters were born, known as Celestia and her younger sister Luna, the wizards of the Royal family wanted to make a group of modified version of ponies under the service of the Royal family. The decided purpose was to make a group of special spy ponies, this group and project were called as Sonitus later. The project was hard to execute, because the wizards needed eight new-born ponies at the same time for the project to be successful. Later after a few months, by the help of the Royal family they were able to gather the given number of new-born ponies, strangely they all were fillies. The parents given their fillies freely and they were not forced to make this decision. They gave them in the hope of a better life among the Royal family. They were two earth ponies, two Pegasus pony and four unicorns.
The little fillies couldn't even move they had no name they couldn't even see the world around them; they all were only just a few days old. The Royal family and the wizards named them - although no one knew in wherein they will be prominent - in order from one to eight: Firoxia, the brave; Fidelia, the loyal; Concia, the hopeful; Tinax, the strong; Risla, the determined; Stratin, the kind, Palor, the wanderer and Pruden, the wise. After some days they were strong enough to stand up and stay still, and the wizards started the second phase of the project. Sadly the process was a little bit cruel and rough to the little fillies. They all were chained down in one place to prevent them from escape, although it was not like they could do anything against it. The wizards started to channel a spell at the same time and a huge blue glowing mark started to appear beneath the fillies, all of them were scared and they started to cry. After many hours of crying the fillies were too tired and later they lost their consciousness and they all collapsed on the floor by one by one. The process took one and a half day without food or water. The Sonitus was officially made.
The spell, which is forbidden now, had advantages but many other disadvantages and side effects. The spell sacrificed their horns and wings to give them alicorn-like immortality and a few special skills, and by unknown reason they were also marked where their cutie marks would be later, this meant that they will never be able to get a cutie marks. Though they lost their horns and wings, by the marks on their bodies they are all able to use magic, and use their specific Air-walk spell which helps them to literally walk in the air instead of flying. The main purpose of the mark was to give them a special ability, Camouflage, with this ability or spell they were able to be fully invisible for over hours and they discovered a side effect, but maybe as a good one, because during this time they didn't breathed. Later this also allowed them to be under water continuously. It's true that they all received alicorn like immortality, but still they are not invulnerable or invincible and they just live as long as an alicorn.
They are what they were made to, the perfect spy ponies in Equestria. Around two months later Celestia was born, and later Luna too. Because the Royal family and the wizards held this project and the group as secret, the fillies were raised as eight sisters amongst the two royal sisters, and they never knew that they had real parents. The Royal family and the wizards always told them that they were made by magic and they had no parents or real family. They used them to spy on other important ponies, at unsafe places or just simply to know what is going on at villages and cities in Equestria. The leaders of the royal guards and other ponies on similar position trained them. During their training they found out, that the spy ponies are more muscular and stronger than an average trained mare.