After Rome fell, the gods began their trek westward, following wherever the world's most powerful nations went. They would focus on powerful monarchy's, which often resulted in having demigod children spread across all aspects of a Monarchy, from kings to soldiers to peasants.
The Twins Mother:
One particular demigod line going through Hypnos and then Morpheus, his daughter Renée de Savigny was power hungry and desperate to become not just any mistress of the new and rather young newly coronated King of France, but the maîtresse-en-titre. The maîtresse-en-titre was the chief mistress of the king of France. It was a semi-official position which came with its own apartments. The title came into use during the reign of Henry IV and continued until the reign of Louis XV.
She managed to hold the spot briefly, but fell out of favour with the king, who only gave her promises to marry her off to someone in his court that had a title. While she waited for the king to fulfill his promises of a suitor, she ended up having an affair with Zephyrus, due to her love for going out for walks on windy days. Within weeks the King finally lived up to his promise of a suitable suitor for her, which was just in time as she was able to pass off the twins as William, Duke of Jülich-Cleves-Berg's biological children. Of course at the time she had no way to know that the Duke was also a demigod, a child of Hermes who had a close satyr friend who came around whenever he ran across demigods that needed training/help.
The Twins Grow:
It didn't take them long to figure out the truth about each other's parentage when they went through a monster attack together. At the time the twins were a few months old, so Renee figured she had a few more years before having to worry about telling William the truth. William spoiled the twins silly as they grew, and Renee continued to manage to avoid the satyr friend seeing them on any of his infrequent visits. She was determined to see her daughter and son be raised in the court. For Jaegar to take on as William's heir and for Cerensa to worm her way into the maîtresse-en-titre position that Renee had lost. She was hard on the children, demanding they never tell William the truth about their powers or to use their powers in front of him. William was amazing to the children and always dotted on them. She two children with William in those first years with him.
A Twins Bond:
As they grew, it was very clear, even when they were still crawling, that they had a close bond to each other, closer than some might say was even normal. A poet might even write a ballad that these two were stricken with Eros' arrow while they were still in the womb and there was no one but them. At first they were allowed to play as they wished together, in any manner, and they went along with things as if girl/boy had no definition for these two. However, as Cerensa slowly blossomed towards womanhood she was torn apart from Jaegar in order to learn to be a proper lady of the court, and Jaegar was pushed into learning manly things like fighting and hunting. Cerensa was always angry when she wasn't allowed to learn the same things her brother was learning. No matter what was thrown at them, no matter how jealous Cerensa got at the things Jaegar got to learn, they continued to grow unnaturally close to each other.
William Goes on a Trip:
This worked ok up until they were around 11/12 years old. Renee was near time to give birth to another child, but William needed to go meet up with his satyr friend to bring in a young demigod who needed training, which with travel time to and fro was going to take about a month or so. With what he thought was just the mother at home that could attract monsters, he prayed to the gods for a blessing to keep her safe from monsters just until she delivered the baby safely or he got home to continue protecting her. (You see back then they wouldn't know exact dates that someone is pregnant for, especially where they had to wait until a missed period to even know they were pregnant. So William had no way to know when she was due or when she could go into labour.)
William made good time on his trip, and was on his way back home, little did he know that he would be too late. The blessing only covered to keep Renee from attracting any monsters. You see Zeus was the one that gave the blessing, and little did Renee and William know, but Zeus was the King's lover, and Zeus never liked Renee much or how she treated the King. He knew many secrets because of his comings and goings to the French Court, including that the twins were actually Zephyrus' children and not William's. He only granted the blessing because he knew that the twins were coming to an age that their scent was getting stronger, and that they alone could easily attract them without the mother's scent.
The Twins First Serious Attack:
And attract them they did, one cloudy and windy day, they attracted the hydra and a couple of hellhounds (which between me and you, I think that Zeus was keeping a close eye on the situation, and when he saw the hydra near asked Hades to send out a couple of hellhounds to round make it worse. Do not underestimate Zeus' utter hatred for Renee and by extension through her, her kids.) At this time it is still 3 to 4 weeks before Renee is technically due. Now Jaegar and Cerensa aren't exactly entirely unprepared for monsters. They do have weapons, they have had some secret training sessions with Renee, but really Renee was more concerned with William finding out the truth, than with actually preparing her children for monsters.
During the attack, as the twins fought for their lives, Renee had no choice but to get up from bed and risk her pregnancy to save her children. The fight was hard, and the twins had minor injuries (and one of the other kids from collateral damage), but Renee took it the hardest, and the stress from the wounds and the fighting she became overcome with pain. William arrived home and learned the truth of things and was grief stricken when his unborn child died due to a miscarriage. Things changed after this, he was no longer the doting father towards Cerensa and Jaegar, he was cold and calculated, always down to business. He also put them through rigorous training to get them up to speed.
This made the twins even closer, which considering how close they already were, there's getting to be less and less boundaries that these two haven't crossed yet, but so far still managed to keep any of the non-typical sibling activities from their parents and half-siblings. Despite her mother's hard work in keeping Cerensa a virgin for the king's pleasure, Jaegar took that liberty himself per Cerensa's wishes.
An Attempt to Separate the Twins:
In the hopes to better their station with the King, when the twins turned 14 Renee sent Cerensa to the French courts alone as one of the Queen's ladies. William, however, decided to keep Jaegar with him to prep him for taking over. Between the duties for the king, his own monster fighting, training and helping other demigods find homes and training, travelling all over Europe, he was aged well beyond his 39 years. Though what he didn't tell Renee, he also suspected inappropriate behaviour between the twins. He hadn't caught them outright doing anything to answer his suspicions, but he'd seen enough to be rightly suspicious. He figured separating them would put a speedy end to anything inappropriate going on.
The twins were, of course, absolutely furious, though deep down Cerensa was pleased with getting to be at French Court. However, she had hoped that her brother would be accompanying her.
The separation only lasted a few months, when Jaegar couldn't stand it any more. He had only gotten one letter from Cerensa and he couldn't stand being apart from her. He ran away and took a lower position than his rank in order to be a guard at the French Court. William was furious when he found out, but as Jaegar had already taken vows, there was little that William could do. So the twins were together again.
Cerensa really only loved Jaegar, but she also loved power and status. She imagined being Queen someday and wanted to go as high as possible just like her mother had. As she wormed her way into the King's confidence, little did she know that Zeus was still making fairly regular visits to the King as well so he was often sneaking in and out of the Castle. Of course Zeus level sneaking is something so sneaky that it would be impossible for mortals or demigods to catch him. This put poor Cerensa at a serious disadvantage. As she wormed her way up to maîtresse-en-titre, following in her dear mother's footsteps, Zeus grew more and more annoyed with her. He started following her and having people watch her.
The King's Bastard?:
By the age of 17 the King was spending more and more time with Cerensa than he was with Zeus in secret, or even with his wife. So when she found out she was pregnant she was ecstatic and immediately told the King. Jaegar was angry with her though, as she told him last even though it could just as easily, if not more likely, be his child as it was the King's. Regardless of who was the father, Cerensa had every determination in the world to let the King believe it was his.
A Bastard is Born:
Cerensa gave birth to a sickly baby boy, in fact everything about the pregnancy and the birth was difficult. Bastien was also born severely underweight and the servants were whispering about that they didn't believe Bastien was even going to live he was so sick. He clung to life though and surprised everyone by living.
Dirty Little Secrets:
For three years every thing seemed like Cerensa really had it all, but what she didn't know was that Zeus was still furious with her and trying even harder to find proof that Cerensa and Jaegar were betraying the King behind his back.
When Zeus finally gathered the evidence he needed, he made sure that it was evidence that would convince everyone of the twins' treachery, the other gods, the King, the mortals, everyone. He finally outed the twins and the proverbial shit hit the fan with such force people were probably feeling it on the other side of the world (yea yea, ok so slight exaggeration.) The King was both hurt and furious. He wanted the twins beheaded for treason, but because the twins came from a long line of Demigods, and Zeus was so heavily involved, the other gods felt that it should be dealt with by them and not the mortals. Because of the King's love for Zeus, he backed down and allowed them to take care of punishments.
Punishment and Curses Oh My:
Cerensa and Jaegar, along with their 3 year old son, were put into a prison cell on Olympus temporarily while their fate was decided. Many felt that leniency should be shown, others didn't care (Hades was one that didn't care, he couldn't even be bothered to show up for the council, just sent along a message with his wife.), and some went along with Zeus wanting the worst punishments possible. After days of deliberation, decisions were finally made.
For Cerensa, because she was both power hungry and sexually hungry, it was decided that she would be forced to take a vow to serve Artemis and join the hunt. To ensure that she couldn't cheat and sleep with anyone the gods made it so that if she did sleep with anyone she would be sick for days after and symptoms could vary from flu like symptoms, to hives, to rashes, to boils. Whatever guys she slept with would be cursed for days afterwards, and there was no telling what their fate would be for those days. The might be turned into a stag for that time, or be sick, or turned into a midget, it was all very random.
For Jaegar, because he was clearly willing to do anything for his sister, no matter what the cost (at one point it even involved killing a servant who had found out their secret, and when a child walked in on them once Jaegar cut his throat so badly that he would never talk again and was in a coma for weeks due to blood loss and trauma), he was cursed similar to how Prometheus was. At some point every night he dies and then lives again at sunrise. The method of death always varies, heart attack, stroke, seizure, monster attack, falling, whatever fate throws at him.
A Special Added Punishment for Both:
Added on to all this, it was also decided that for around 500 years, the twins would be unable to see each other. If they got within 100 or so yards from each other (close enough that sometimes they could see each other but not really talk without serious shouting), they would be automatically teleported 500 miles apart (with the destinations being totally random). Which in the last 500 years has happened on quite a few occasions, especially early on, with the twins trying to see each other any way.
Bastien - Collateral Damage:
Zeus and a few seemed to have zero interest or empathy as top what Bastien's fate would be. However, he did not want the twins to know what was decided for Bast so the twins were cursed and sent away from Bastien. By the age of three it had been clear that he was a midget, and would be lucky if he ever got to be 4 feet tall. It wasn't his fault that his parents were twins that did bad things. Demeter and Morpheus went up to bat for little Bast, insisting that he be taken care of. Zeus told them that they could have Bast, he only cared about punishing Cerensa and Jaegar. Morpheus turned little Bast into a Dream Spirit, and then so he could continue growing and learning, Demeter sent him to live with one of her Children who had married a Sleep Nymph. Once Bastien was full grown and ready, he went to serve Morpheus. Though over the 500 years, he would often go "home" to the only parents he had ever known and spend time with them. He also took a great interest in the mortal world, way more than in dreams and sleep. He turned out to be quite the opposite you'd think of for a Dream Spirit, but he never let it show around Morpheus. He might have been only 3 when all the shit happend with his parents, but he understood what Morpheus did for him and he was grateful.
A Secret to the Curse:
One thing that no one besides Zeus, Morpheus and Demeter know about, is that there is a way to end the curses on the twins, but only through their son Bastien.
Camp - 2014:
Periodically over the years all three have been at camp at one point or another. Of course the twins never too close to each other if they did manage to be at camp at the same time. Now that the part of the curse has lifted for the twins not being allowed to see each other, they both wanted desperately to get to camp. When Bastien found out his parents were headed there, he wanted to see what happens and decided to go as well. Demeter did tell him that the twins weren't told about him being turned into a dream spirit the day of their sentence, and that she had no idea if at any point in the last 500 years they had found out what happened or if they even cared. This made him even more curious about what would happen.