The Holy Grail of Yggdrasil

The Holy Grail of Yggdrasil is a miraculous relic capable of making any wish come true. It's the object that's at the center of the Holy Grail War of Yggdrasil, the reason why the conflict breaks out.

Origin
The Holy Grail owes its magical powers from Odin's "sacrifice". As written in the Hávamál:"' I know, I was hung on the trunk whipped by the wind for nine whole nights, wounded by spear and given to Odin, myself to myself, on that tree that no one knows where from the roots rise up. With bread they did not satisfy me nor with horns I quenched my thirst. I looked down, got the runes up, called them, and fell over there. '"In the Age of Gods there were Druidic cults that venerated divinities and nature, Odin gave thirteen druids a sacred object that contained his blood and that could fulfill any desire (the Grail), apparently it was a gift made for reward the loyalty of the druids to the gods. Loki, however, managed to convince the druids to shed the blood of the Grail on the ground to do a ritual that would make them as powerful as the gods, however the blood did nothing but attract Nidhoggr.The druids managed to prevent Nidhoggr from devouring the world and sealed him in the Grail thanks to a ritual, but there were serious consequences.

Eternal return
As Yukiko explains, Nidhoggr is not a demon and not even a spirit but a divinity and when he was imprisoned in the Grail, the druids gave rise to a paradox. Whenever the Holy Grail is used, Nidhoggr comes out, destroys the universe, but then recreates it. Nidhoggr destroys and creates. This paradox is due to the miraculous characteristic of the Holy Grail itself, which made Nidhoggr a divine snake capable not only of bringing death (as it is its nature to do) but also of creating a new universe.

According to Yukiko Nidhoggr became an Ouroboros. The problem is that this eternal return (which is inspired by Friedrich Nietzsche's philosophy) is not perfect. Every time a new universe is born something changes, the new universe is always different from the previous one and therefore the conditions themselves that led to the end of the world also change (for this reason one could theorize that there has been more than one Holy Grail War of Yggdrasil). Another feature of this "eternal return" is that the Holy Grail is excluded every time from the rewriting of the new universe, remaining an unchanged presence in every universe.

From Yukiko's explanation it can be deduced that if the Grail is used in a certain year and the world ends, in the new universe it can be used in the same year as people's actions are repeated in every universe (probably because they're not aware of the previous universe) however this is avoided only if there are enough alterations to prevent the Holy Grail from being used in that particular year. All this is possible because each new universe is not an exact copy of the previous one, there may be imperfections.

The ritual
Apparently it's not easy to activate the Holy Grail of Yggdrasil, first you need to do a ritual in a point where there's a magic knot (such as the Westminster Abbey) and then the one who wants to do the ritual must possess a thorough knowledge of Nordic magic. Then you need a piece of Nidhoggr's skin, which must be buried underground. Finally, the blood of at least one descendant of one of those druids who dealt with Nidhoggr is needed.

The Coven
In the First Arc, Marco Lombardo reveals to Alessandro that Marco Serpi intended to evoke Nidhoggr using the Holy Grail. However in Lombardo's explanation it's understood that the Coven, although knowing the Grail, doesn't have the exact knowledge about it because Lombardo himself says that the Holy Grail was evoked by the druids and therefore also Nidhoggr (it can be understood that he's wrong because he has no idea how it was possible for them to do the ritual without the tools proving that they, the Coven, are not really sure of what happened).

Nidhoggr's skin
It's not clear where it comes from or why it's there but it's known that it's useful to do the ritual to activate the Holy Grail. Apparently the skin was preserved by Celtic tribes inside a small golden coffer. In 1877 Schneider von Bergedorf discovered Nidhoggr's skin and began to study it. The coffer was then confiscated by the Nazis and then was taken by the Soviets and disappeared. The Rote Mäntel sought the coffer in Russia but without success.