Stargle Poison
A stargle fruit is a fruit that harbors the same magical essence as the serpents that crafted the world itself, celestial magic. The fruit is a boon to those with celestial or light magic and is quite nutritious for the other natural magic types as well since all magic stems from celestial magic. All magic except dark magic, however.
Dark magic goes against the natural laws of the world and corrupts all that it touches. Because of its innate characteristics, stargle fruits, which contain celestial magic, naturally purify dark magic since it is the direct opposite.
There are some who take advantage of this. In particular it would be demon hunters or jealous demon nobles. Demons themselves are beings that have had their souls tainted with dark magic, be it by they did something so horrid and against nature it can naturally or because they came in close contact to it in a past life remains to be seen. Because celestial magic purifies dark magic, stargle fruits are poisonous to demons.
Stronger demons, such as the Demon Princes, can eat stargle fruits with no issue, but weaker demons can die from a single bite of the fruit or a sip of the juice. A subversive use of the stargle fruit is to give weaker demons trace amounts of it to kill them. This is also used for some stronger demons as well but it does take more of the fruit or a higher concentration in order for it to be effective. There is a trace of celesi magic left on them, but even if they were to find the trace, the only people who grow stargle fruits are the Demon Princes and no one dares to try to go against them out of fear they may be killed outright for trying to investigate in the castles of the Princes.

The idea that something good and pure can be used as a poison is really interesting. I love that it makes perfect sense for why it is a poison for demons but the opposite for others. I was wondering if the stargle fruit can be used to purify other things? Such as an item or something that has been tainted by dark magic, or if the fruit only works if consumed. Also, does the fruit give special benefits to others or just extra nutritional value? Anyway, I thought the entire concept of the article was great!! Feel free to ask if you need any clarification.
The fruit is saturated with the celestial magic, so the flesh of the fruit or the juice (or anything made from it or the leaves/bark/wood) also has celestial magic. An item with dark magic that has been soaked in stargle fruit juice would have the dark magic purified if the dark magic itself isn't too strong or concentrated. The fruit, when given to someone who doesn't have dark magic, does have an increased rate of recovery for their magic and their magic is stronger for a shorter period of time. Celestial magic is the root of elemental magic, thus why it bolsters elemental magic. The fruit doesn't have to be consumed to harm either, such as making a syrup and coating a blade with it is a popular method for assassins in the Demon Prison