i'm making this blog suit to a comment i made before of a theory i made about danmachi background, just gonna copy it here:
though if i'm allowed to speculate, the thing at the buttom of the dungeon is probably what ouranos been praying at, to keep calm the dungeon.
i'm gonna go wild with speculations, i think it's possible that at the buttom of the dungeon might be a god, who was banished from heaven longtime ago. maybe few thousands year ago, when the dungeon 1st appeared on earth,he/she would be the one that created the dungeon, and continuesly, repairing and spawning monster inside of it using its arcanum powers, since the rules of the gods dont apply to him, he didnt decend after the agrrement not to use arcaunm on earth was made by the gods, so it doesnt affect it.
the reason why , when asked about the dungeon, the gods dont give a straight answer, is because they dont want talk about the exiled god at the buttom of it.
they also say that the dungeon is a dungeon (or the dungeon), probably because that's what it is, some sort of prison cell.
that also explane why the dungeon hates the gods, and reacts violently to their presence by spawning black type monster. like how it reacted to Hestia and hermess. since the exiled god still hold a grudge againt the other gods for locking him up.
maybe that Oath fels mentioned to bell on volume12, is some sort of deal made by this exiled god, that if a child, reaches him, he'll forgive the gods.
also, just to spice this up, let's borrow some biblical references, the more crazy it gets. the funnier, let's say the exiled god's sin, was that he opposed the creation of the children races (humans and demi-humuans), and didnt see them worthy of gods and godesses love and attention. and so he got exiled down there. or maybe only humans, since they seems special, , specially humans that can copulate with other races and make half-demi-humans, what an abomination.
a thousand year ago, this god became more powerful, and managed to create the 3 legendary monster, leviathan, behemoth, and black dragon, when the other gods realised this, they decided to go down to earth and help the children by giving them falna, at the pretence of seeking entertainment but the realy rason was so they can protect themselves& the children. that's probably why the final goal of the adventurers is to defeat those 3 beasts.
still the exiled god wont rest, until the oath is fulfiled and the children reach the buttom of the dungeon he created, to prove their worth to gods. to prove that they can become powerful enough to rival them.