The Light Novel series is the source material for the adaptation of the Anime and Manga series. The information and terminology on the article will be based on the source material rather than the adaptions.
Jureve (ユレーヴェ Yureeve) is a type of magic potion used to heal nobles from a near-death state.
A Jureve is a potion that usually every noble will brew in the course of their lives, since it is a life-line that can cure what otherwise would be fatal injuries and illnesses. Typically a noble will start to gather the necessary ingredients once they enter the Royal Academy and most nobles finish this gathering during their time there. Laynobles with their low mana capacity generally have a much harder time getting the necessary ingredients together and dyeing them in their mana, thus take the longest, some not even being able to finish until well after their graduation.
Spoiler from Part 4 Volume 8
The recipe for the jureve is taught in the fifth term at the academy.
Upon completion, a vial of the potion is always carried at all times, so it is at hand should an emergency happen.
Spoiler from Part 4 Volume 8
Generally the higher the mana capacity of a noble is, the higher quality the jureve needs to be to properly work for them.
If the potion should become necessary, a small portion of it is drunk directly, while the rest is used to fill a tub to submerge the entire body in it. When the noble enters this Jureve bath, they will fall into a deep sleep as their body enters a state of suspended animation, where all bodily functions are reduced to a minimum. For the duration of this sleep, they do not require any form of sustenance and will not age, their bodies however keep producing mana, so it is necessary to have someone regularly drain the excess mana, should the sleep last for a long time. This can be done by simply putting empty feystones into the potion's liquid and withdraw them after they have filled up. It is also worth noting that the bodies are not frozen in time. While the aging-progress is halted entirely, the muscles will decay and atrophy at the same rate they would in a regular coma. The healing properties of the potion can not offset this. Furthermore should the person still be in their growing period, unlike the rest of their bodies their mana vessel will keep growing.
Once a noble enters a Jureve-induced coma, there is no way to wake them up. Those taking care of them will have to wait for the process to run it's course and the person to wake up by themselves. Depending on the state of the afflicted person this can take anything from days to month or even years. The longest recorded case of a Jureve sleep was Lady Rozemyne of Ehrenfest, who spend almost two full years in a Jureve.
Apart from the ability to heal injuries, a Jureve can also dissolve clumps of hardened mana. When a mana-wielding being is near death or clinically dead, their mana will flow back to their mana-storing organ near the heart and crystalize there. Should the being recover, these clumps will cause problems with their mana-flow and need to be taken care of. The longer they remain, the more difficult they become to dissolve, requiring both more time and a Jureve made of higher quality ingredients.
Rozemyne has to make jureve to melt away the mana clumps inside her body. As years have gone by since the clumps manifested, she needs materials of the highest quality. Those are gathered throughout the year in each season when the seasonal mana is the strongest. If she fails to obtain one material, she has to wait a full year until she has the chance to gather it again.
Once she successfully gatheres all materials with the help of her family, guardians and retainers, as well as the assistance of the entire Knight's Order for the winter ingredient, she is able to brew the potion, but due to Ferdinand expecting that the Jureve will put her to sleep for several month or maybe even up to a year, they hold off on using it right away, so Rozemyne can hold her sister Charlotte's baptism ceremony as she promised and participate in winter socializing.
Unfortunately during the celebrations after the conclusions of Charlotte's baptism, Rozemyne is attacked and poisoned by an unknown noble, leaving her near death. This forces Ferdinand to put her into the Jureve right away to save her life.
Due to how long the mana clumps have already been in her body and the effects of the poison, she remains asleep for one or two month shy of two full years. Unfortunately due to also needing to heal the damage done by the poison, her mana clumps are not fully dissolved, leaving her considerably better off than before, but not entirely healed. Furthermore on account of her unusually long sleep her muscles have atrophied to the point of not even being able to list the weight of her own arms without the use of magic tools or enhancement magic.
Spoiler from Part 4 Volume 8
Late in spring after finishing her second year at the Royal Academy, Rozemyne makes another jureve and immediately uses it. This time she is only asleep for four days and the potion is able to dissolve all of her remaining mana clumps, in theory opening her up to finally grow at a regular pace and become fully healthy.
The main ingredients of the Jureve are one feystone of each seasonal mana, green for the waters of spring, blue for the fires of summer, yellow for the winds of autumn and white for the snow of winter. The quality and thus the strength and efficiency of the potion depends on the purity and the capacity of the feystones, as well as how much mana is poured into them.
It is absolutely necessary that the entirety of the mana within each feystone has been fully dyed by the person who the potion is being made for and that they also do the brewing themselves. Noone can make a Jureve for someone else. If someone intends to aid the person who is making a Jureve, it is necessary to take strict precautions to avoid accidentally contaminating their ingredients and potion with your own mana by accident.
In addition to the feystones several herbs and oils are used to finish the Jureve.
The following components are of the highest class and were used to create Rozemyne's Jureve. Using different ingredients is possible and in fact common, but will result in the final potion to be weaker than the one used by her.
- Ruelle - This fruit of the feytree of the same name can be collected near the small city Dorvan on nights with full moons. It is particularly potent when gathered on the Night of Schutzaria in autumn. Special care should be taken, since the mana concentrated inside the fruits on the Night of Schutzaria is so strong it lures large numbers of feybeasts to the trees and sends them into a feeding frenzy, making the particularly dangerous.
- The Lord of Winter's Feystone - This feystone left is left behind by a slain Lord of Winter. For the stone to reach it's fullest purity and capacity, it is necessary to kill the beast by attacking it's feystone directly, which comes at the cost all loot save the feystone itself.
- Nectar of a Rairein (ライレーネ Raireene) - This nectar of the rairein flower, collected on the Night of Flutrane at the Goddesses' Bath.
- Riesefalke Egg - The egg of a big and dangerous winged feybeast called the riesefalke. They nest in the caverns of the active volcano Mount Lohenberg in the Ehrenfest province of Griebel.
Components from spoiler content
Spoiler from Part 4 Volume 8
- Flammerzung Fruit - The fruit of this feyplant is of exceptional quality, but in comparison to the ingredients used for Rozemyne's first jureve, lacks the necessary purity to be used right away. To become usable for a jureve, the brewer needs to remove the traces of other elements from it before dyeing it. The primary element of Flammerzung is fire.
- Quellweide Leaf - The leaves of the feyplant Quellweide are a high quality and valuable ingredient, albeit they lack the purity of the ingredients of Rozemyne's first jureve, meaning the brewer has to remove the traces of unwanted elements from them, before dyeing them with their mana. It is rich in wind mana.
- Winfalke skin - Skin of the Winfalke is a valuable and high quality ingredient albeit lacking the purity of Rozemyne's first jureve, making it necessary to remove the unwanted traces of different kinds of mana before dyeing them. It is rich in earth mana.
- Glanzring Dust - This dust is a valuable and high quality ingredient, albeit not entirely pure. To be usable for jureve the traces of unwanted mana need to be removed before dyeing. It is rich in water mana, but it's currently unknown what kind of creature or plant the dust is harvested from.