Know your lore #1: The underworm and the underdark

Time to understand what the hell I'm throwing at you. Party members might be wondering what exactly is going on, before forgetting why they were wondering about that when they need to face down some serious monsters.

Though Gears of War 2 had a hand in inspiring the layout, I swear I thought of this concept a few days before finding out about that level. At any rate...

The underworm is a massive creature that most scholars have never seen, let alone heard of. Because it lives underground as if it were the depths of the deepest seas, few have ever seen it in it's entirety.
It eats whatever it can swallow with it's maw. It'll even break down earth for the minerals when it can. It's digestive juices will break down anything, but the time it takes to do so varies from food to food.
It has a blowhole similar to cetaceans, which it uses to take in air whenever it's in short supply deep underground.
The underworm tends to be passive when it comes to food. It doesn't actively hunt per se, but it does employ ambush tactics and surprises to swallow most of it's prey whole.

Two main monster attractions within is the giant heart and the white blood cells.
The white blood cell is of course, the main bulk of self-defense within the underworm. Their first and most used tactic is to simply slide on their way, and engulf whatever is in it's path. If anything's left after it swallows one creature, it relies on slamming it's body, particular with the extra momentum of a victim, to subdue other intruders until it digests it's engulfed target.

The giant heart can't move, but it's has a long reach, and has a number of reaction mechanisms to protect itself from harm.

Just like the surface, the underground is brimming with life. Many communities are built underneath the sun's reach, and just like the surface, their are adventurers underground too. The most common adventurer underground is an explorer, usually a cave cartographer. One who makes a living charting the twisting pathways and caves so future travellers don't get lost or run into dangerous creatures. Lighting the darkest reaches are settlements meant to act as safe havens for these explorers, usually founded near an exit point to the surface.