Where did the Onyx Deck come from?
 

Where did Onyx come from?

It was never fully explained where the Onyx Deck came from. Since there was no Ventarian counterpart. Kit just had nightmares about Onyx , only for the Advent Master to give him the deck.

Perhaps it was a deck that the Advent Master had just constructed before his original battle with Xaviax. Maybe he somehow always new Kit would become Onyx? It also could have been a fail safe in case something happened to the Dragon Knight Deck.

Or perhaps Eublon actually had the power to summon it from Kit's nightmares? But that seems to contradict the explanation that the Advent Beast was appearing to Kit in his nightmares. But perhaps it was really the Dragon Beast we saw in Dragon Knight. And Eublon just used the nightmares to create the new powers.

Re: Where did the Onyx Deck come from?
 

It was possible that the Black Dragon Advent Beast appeared in Eublon's dreams during his years in he was sleeping on Earth. It could have given him the idea of creating a Onyx Advent Deck, which he went to create after he woke up, That's what I think. ;)

Re: Where did the Onyx Deck come from?
 

It came from where all Advent Decks came from Japan.