Two thousand years ago Tyr was just a sacred wood. At some point the area turned to swamp land. Once Kalak came into power the city was built a top the swampland. Pillars were erected to hold up the streets and Under Tyr was born.
Now it is akin to a vermin filled giant dungeon where thieves hide from the templars.
While Under Tyr is not itself a naturally made environment it does have inhabitants. Escaped slaves, Veiled Alliance, thieves, scavengers, and even templars will enter the undercity in order to avoid being seen. Many adventure into Under Tyr looking for ancient tomes and hidden temples that may still be there.
Many creatures reside within the underground areas. Most common are insects and undead. There is a healthy amount of Id Fiends that also call the city home.
The Crimson Shrine
A sacred shrine that used to be a temple 2000 years ago. It was erected to the worship of an eagle headed deity that brought rain, whether such a deity actually existed is up for debate. What is known is that the shrine is protected by Crimson Knights, wraiths covered in steel armor that wield halberds. Per their faith only the pure of heart may enter the shrine, all others are barred from entry. Unlike many creatures on Athas they will give warning to tresspassers, but will fight to defend the shrine if they do not head the warning. Inside the shrine are over a thousand candles lit by the faith of the Crimson Knights. Whenever a Crimson Knight goes against its faith a candle goes out. No candle has ever been extinguished.