Tellus: The Underworld or, rather, Afterworld

Its basically near mandatory for D&D-related settings to have vast underground complexes to justify all the monsters and other things found deep inside the Earth. Tellus is, of course, no different and has an Underworld. Generally speaking, knowledge of the Underworld is limited: there is mostly only folklore and myths to go off as well as concrete knowledge of particularly large caves networks inhabited by monsters. The truth is a bit stranger.

The Tellusian Afterworld

The plane of Esper from Magic: The Gathering

The mythic underworld of Tellus is best understood as being two or more layers which somewhat intersect. The surface layer itself is what one would expect: it is a series of semi-connected caves, often areas of geomantic potency. Usually these are relatively large cave complex with running water connected to underground sea and odd local wildlife. There are many 'dungeons' in Tellus at least partially connected to these, namely because generally dungeons are ruins and ancient ruins have a tendency to have been built in geomantic locations to begin with.

While Tellus doesn't quite have Outer Planes, if one goes down enough, down the deepest chasm and the oldest ruins they will eventually transition into a more pocketed dimension, which is both inside the very surface of Tellus and yet not quite. There has been a lot of mentions here and there of Tellus taking cues from many concepts found in other JRPG (which themselves derive from mythology and philosophy), such as the Lifestream. The Afterworld of Tellus is akin to a mixture of an outer plane, the past and the akashic records made manifest. This Afterworld can be the setting to an entire campaign or, more likely, the type of map which would reveal itself in the final acts of a JRPG. In there are entire domains which are the distorted perception of past eras and zeitgeists. Possible use for the Afterworld (or one incarnation of it, anyway) in a game include:

• The domain from which an ancient evil, long thought defeated, operates from. This force would be from a world that is a horrid, twisted and broken place as a reflection of its cursed desires. In this case it would be the 'trippy final dimension' where the boss is fought.
• The idyllic yet haunting manifestation of some antechamber of the dead, where the spirit of a long dead hero could be beseeches for its guidance, to borrow its ancient artifact (or find the location of its tomb to obtain said artifact.
• The Afterworld could be, at least in one specific incarnation, a secondary world brought into conflict with the surface Tellus. This Afterworld would embody either a lost 'golden age', reflecting on the current degraded state of modern Tellus. Alternatively it could be a more brutal, barbaric place filled with nothing but contempt for the current one.
• The last home of a dying race currently gone from the current era of Tellus.
• As a source of ancient, long lost and forgotten technology.
• As an alternate setting.
• Or as something as close as possible to an Outer Plane as Tellus has, but that's a bit boring.

It should be noted these echoes of the past cannot be reached through simple spells available to players. These require physical travel or artifacts to travel to and from. If these ideas seem a bit too bare bones, I won't lie that they are still in their infancy and their full potential hasn't been tapped into yet.