In this blog, I'll elaborate on my theory concerning Lumen Histoire.
After thinking about the possible nature of Lumen Histoire, I arrived at the conclusion that Lumen Histoire may be one of the Books of Zeref or that it is derived from Zeref's Books.
So far, only a few comments have been made about Lumen Histoire, so let's analyze them.
Makarov stated that it was the Guild's "Darkness" and even called it the "dirty secret" of FT, while also revealing that it was what Mavis wrote about the true nature of their Guild. This really intrigues me as Lumen Histoire was called the Guild's "Light" by Mavis.
The possible explanation I can think of here is that Makarov doesn't know all there is to know about Lumen Histoire and that he just knows that Lumen Histoire is in some way connected to Zeref, perhaps due to Zeref's name being mentioned or his symbol being shown in Lumen Histoire.
If he doesn't know the true meaning, it seems normal for him to deem something connected to Zeref as "Darkness" and a "dirty secret", since it could get FT into trouble if the Magic Council (which was still functional when he made the comments) were to find out about FT being connected to Zeref in one way or another. Given their hatred of FT and fear of Zeref, they may disband the Guild there and then.
Since he may be aware of the connection between Lumen Histoire and Zeref, he could have deduced that Lumen Histoire was in some way a catalyst for Precht's fall into darkness. Makarov may understand very well that Lumen Histoire could potentially tempt those seeking power and cause them to fall into darkness.
Now, let's see why Mavis called it the Guild's "Light". For starters, she wrote/created Lumen Histoire, therefore it is pretty straightforward to say she knows all about it. As such, she knows the purpose behind the existence of Lumen Histoire.
The fact that she called it the "Light" suggests it has some benefits to the members of the Guild. My prediction is that it contains details concerning Zeref's power, origin and a possible way to stop him.
Several clues have been given so far which could suggest these contents.
The first would be the simple fact that Zeref and Mavis know eachother, thus Mavis has interacted with Zeref during her lifetime and likely witnessed his magic abilities first-hand and probably discovered quite some information about the legendary black mage.
Something I noticed is the fact that Mavis' spirit appears to be about the same age as the Mavis shown in Walrod's flashback of FT's founding. This makes me assume she died not too long after founding FT, possibly at the hands of Zeref. Judging from the conversation they had after the GMG, they had been involved in fighting in the past as Mavis asked Zeref whether there would be fighting after Zeref revealed he wanted to exterminate humanity. During the same encounter Mavis states to Zeref that FT will stop his "one-sided annihilation", all this information made me come up with the following explanation of their interactions during Mavis' lifetime:
Zeref and Mavis were involved in a war. Zeref was stronger, but due to Mavis strategical expertise (being renowned as the Fairy Tactician) she and her allies managed to survive and counter Zeref for a while.
Being a tactician, she would look for information regarding her opponent in order to find a way to defeat him. As such, she researched his Books to find countermeasures to his magic. Given the fact Mavis wielded some impressive magic herself, she may have rigged one of his Books to one day help the holders of said Book overcome him.
Eventually, she compiled all the information she obtained as well as a strategy to eventually beat him into the rigged Book and named it Lumen Histoire.
Shortly after, perhaps while trying and failing to execute her strategy, she died while confronting Zeref and now she hopes the current generation of FT will be able to do what she couldn't.
With Lumen Histoire providing FT a possible way to fight Zeref, it could certainly be called a "Light" that will guide or save them and it would also mean that the true nature of FT is that they are the only Guild being able to permanently beat Zeref.
Another potential clue linking Lumen Histoire to a Book of Zeref is Precht.
He was around when FT was founded, thus he knew Mavis and possibly encountered Zeref as well. After succeeding Mavis, he may have studied Lumen Histoire and upon reading what was written about the origin of Zeref's power, he became obsessed with obtaining the same power.
If Lumen Histoire is a rigged Book of Zeref which still contains Zeref's original writing, this may be where he learned to create demons from pebbles and to create/obtain a Demon's heart and eye.
Bear in mind that a "grimoire" is a magical book which contains all sorts of magical stuff (summoning, charms, divination, spells, creating talismans and amulets, etc). The fact he named his Guild Grimoire Heart may suggest he learned how to create a "heart" by reading a "grimoire" and said "grimoire" could be the current day Lumen Histoire.
Perhaps Zeref's original writing detailed how to cast Grimoire Law and this inspired Mavis to create Fairy Law, which may be a less cruel/dangerous version of Grimoire Law, but still very potent in it's own right.
However, Precht preferred the original spell created by Zeref. Once again the word Grimoire should attract attention, likely meaning that the spell was found in a "grimoire".
Sure, he may have learned the Nemesis spell from another book and obtained the eye and heart through different means, but does it sound logical he encounters a Book of Zeref which contains a spell quite similar to one of the Great Fairy Magics? I beg to differ.
It may even be safe to say the Three Great Fairy Magics were derived from Zeref's magic in order to damage him, in the cases of Fairy Law and Fairy Glitter or protect against him, in the case of Fairy Sphere.
The final clue would be Obra. Since Zeref likely knows what Lumen Histoire is, he probably was waiting for many years until someone (Natsu?) would use it against him.
However, now that he wants to annihilate humanity rather than just waiting to be killed by Natsu, it would be a hindrance rather than a blessing if Lumen Histoire would be used.
Therefore, he contacted Ivan (the person most likely to know how to obtain Lumen Histoire, given he was a member of FT and the son of the current Master) and offered to bolster the ranks of Raven Tail by lending him Obra in exchange for Ivan tracking down Lumen Histoire.
To make a long story short, I believe it's certainly possible that Lumen Histoire is a Book of Zeref which was rigged by Mavis in order to one day provide FT with the necessary means to combat Zeref.
The fact that Zeref's own magic would be used in a strategy to beat him makes sense and it would be similar to the Dragons teaching their own magic to Humans, since only a Dragon's magic can injure or kill a Dragon.