Post-Apocalyptic High Fantasy:
This realm used to be a beautiful and thriving classic high-fantasy setting.
However, this idyllic peace was shattered by conflict.
The classic debate of "magic vs technology" had become violent. Extremely violent.
Great spells of devastation crossed fire with the most powerful weapons that could be created, scouring the surface of the entire planet.
The best bets for survival were underground, extremely high altitude, or in the depths of the sea. All the beings that could flee, did.
All of the various shallow-earth beasts and cultures were forced into the deeper depths, forming uneasy alliances with beings that had already lived down there.
Dwarves, shallow-earth dragons, deep-underground dragons, and mantle-dwelling fire dragons, put their differences aside and worked together to build a sprawling network of underground tunnels, weaving through the depths of soil, bedrock, and the upper sections of the mantle for those who can handle the heat.
Oxygen for this network and its denizens are provided through a mutual relationship with oxygen-fixing microbes. Some of the dragons have ended up with these beings in their bodies.

Terrance hatched from a small clutch in the soil region of the cave network, in a den carved out near a local dwarf village.
His mother taught him how to use his abilities and hunt for food, and how to interact with their society on a basic level.

As Terrance became old enough, he and his siblings joined schooling with the Dwarf village, where they received a healthy and well rounded education.

Once graduated from the relatively strictly regimented Dwarf school, Terrance set out to explore the rest of the cave system.
He made various friends with other varieties of dragon, as well as neighboring dwarf settlements.

By age 35, Terrance found that his local region has begun experiencing famine.
He volunteered his abilities to hep find and retrieve food for the subterranean community.
Thus forged Terrance's day to day life, rippling through the dirt zone searching for various beasts to hunt and retrurn to the cave networks by "day", and then coiling up in a cozy abode by fireside by "night" to share tales of adventure.

Terrance was out hunting, when he noticed a disturbance in the soil that wasn't there before.
His sonar ability couldn't make out what it was, so he turned and headed that way to investigate, sending himself through a breach and into the realm of Infinitas.