The Lands Beneath

The Lands Beneath is a dimension located below the Mortal Realm's dimension
It is also referred to by the Mortal Realm as 'The Underworld' or 'Hell',
despite it not looking remotely like human's depiction of 'hell'.

The visuals of The Lands Beneath are similar to that of Earth but with far more massive and varying bioms,
radical weather and a completely different social construct. The superior species existing in this dimension are 'demons'.
Demons here have similar traits to the demons depicted in Christianity but extended abilities, less weaknesses to 'angels'
 and are very much less focused on tempting, antagonizing or hunting humans. They simply have better things to do with their lives.
In fact, there are many demons befriending humans or even having intimate relationships with humans.

Demons bypass time, thus making them not age and be able to cross dimensions without facing the troubles of taking years to reach their destination
if they were to travel without use of their portals. (Example: with a spaceship or some other device that takes longer than their demonic portals)

Other creatures aside from demons also are birthed from and reside in The Lands Beneath, a lot of them looking like 'pre-designs' to existing animals in the Mortal Realm,
like bats for example, there are massive bats in The Lands Beneath and they aren't the only 'oversized' animals in this dimension.