Theory related to Norse mythology:
Basically a few random connections I found...
(I might be completely wrong but I feel it's interesting)

Megicula / Vanica and Hel (Norse myth)

(I will skip William's magic and Yggdrasil since i feel its obvious...)

Hel is the daughter of Loki in Norse mythology and rules over 2 worlds connected by Yggdrasil: Niflheim and Helheim (aka Hel).

Vanica is a dark triad and rules over spade kingdom and Megicula is probably a high rank devil and might control the underworld.

Niflheim and Spade kingdom:
Both are described by similar words like coldness, winter, mist ,ice ,etc.
Spade kingdom is the northern kingdom and Niflheim is also mentioned to be north (?) (I dont really know about this one)

There is a serpent/dragon called Nidhogg gnawing at the root of the tree near Niflheim and is literally called "CURSE striker" or "he who strikes with MALICE".....doesn't that sound familiar?
Julius also mentions "ancient horror which slumbers" which might be related to this

also, Hel is also the sister to Fenrir (wolf) which many people theorize to be Charmy's wolf (i dont know how that fits into picture in black clover)

Helheim and Black clover's hell:
seems like Helheim is a dark and cold place...

We don't know much about black clover's hell
but Yuno describes Zagred's mana as cold and we see Lucifero stuck in something like ice so I guess it might be a cold place.

even the centre of hell in Dante's inferno (from Dante's divine comedy which is another inspiration for this arc) is cold so I feel it supports that black clover hell is also cold.

Helheim is also the place where people who die from old age / sickness go.

In black clover we see that the elves were going to send the human souls to the underworld in order to get fully reincarnated.

i might add more stuff when we get more info