Could someone possibly explain the Sub terra for me? As in what it's for and how to get through it.

Its mainly a training place for high levels, there's a couple of hunts there but nothing hugely significant.

There's four floors (though the last one is only accessible twice). You can unlock these floors by collecting Black Orbs and putting them in the Pedestals found in the four dark rooms on each floor. Bear in mind that only the monsters found in these rooms drop the Black Orbs. You can put 1, 5 or 10 Orbs in a Pedestal at a time. However, if you put more Orbs in then required, they will go to waste.

Pedestal North-East North-West South-East South-West
Penumbra 18 9 3 6
Umbra 15 9 15 18
Abyssal 15 21 27 12

Clearing the Third floor (Abyssal) will grant you access to Hell's Challenge. Here you will fight the Elite Mark, Shadow Seer (Hunt: God or Devil?)

Hope this is helpful =).