Here is the official Underfoot expansion announcement as seen on EQLive:

Announcing EverQuest Underfoot!

In November 2009, the inhabitants of the world of EverQuest will enter the realm of Brell Serilis as they explore through the Underfoot. Announcing EverQuest® Underfoot, the 16th EverQuest expansion!

The Underfoot centers around the Heart of the World. This is the vast magma ball that floats in the center of a hollow sphere many miles across. Standing on any open interior portion of the sphere characters can look up to see the massive, blazing ball of living lava. The sphere is large enough that while individual areas have a "bowl" shape it is very minor. Lava flows into and out of the Heart of the World from volcanoes, lava tubes and crevasses all around the hollow sphere, like blood into and out of a heart.

It is into this troubled realm that players will travel.

Features:

Along with the many new quests, items, spells, and alternate advancement (AA) abilities, Underfoot will include the following features: 

• Expansion Progression Window – You'll be able to easily see what you have accomplished through Underfoot and which objectives you should achieve in order to travel in your desired progression path.
• Focus Target Window – This new target window will let you add specific targets to your list which will be helpful to monitor the health of Friends or Foe.
• New Zones – 12 to 15 new zones.
• All Inclusive - The expansion includes all features and content from the 15 previous expansion packs.
• More Weapon Models - Underfoot will have twice as many weapon models as any of the previous 15 expansions.
• Visit the EverQuest Underfoot website for more details.

Background Lore:

The Plane of Underfoot

The Plane of Underfoot is ruled by a curious being known as Brell Serilis, the Duke of Below. Brell is dwarf-like in appearance, wears the garb of a spelunker, and wields a magic pickaxe. The plane itself consists of an elaborate system of caves and caverns. This plane is inhabited by all sorts of creatures, both good and bad, that normally dwell under the ground. Brell Serilis created the dwarves and placed them beneath the surface of Norrath as part of a pact among Tunare, Prexus and himself, in response to the arrogant claim staked on Norrath by Veeshan.

Brell Serilis

Brell is allied with Fizzlethorpe Bristlebane, and an enemy of Veeshan's. Followers of the Duke of Below believe that the surface world is a waste of space. They find true happiness in the caves, caverns and tunnels that permeate the belly of Norrath, but this is one of few points upon which all followers of Brell can agree. There are many different and often opposing factions who worship Brell. The Runny Eye Clan of goblins claim he is their father, much to the disgust of the dwarves of Kaladim who are certain that only the dwarves are the true children of Serilis. The vicious gnolls of Split Paw disagree entirely, for was it not Brell who sculpted them out of the sacred Clay of Cosgrove? Followers of Brell Serilis, in one form or another, can be found nearly anywhere you enter the Underfoot of Norrath.

History of the Underfoot

In time the other gods noticed Veeshan's work, and being often petty and jealous beings, they too came upon Norrath, intent upon leaving their mark. Brell Serilis was first, and from his Plane of Underfoot, a dark realm of vast caves and endless tunnels, he quietly created a magical portal to a cavern deep in the belly of Norrath. Through this portal the Duke of Underfoot seeded the depths of Norrath with all manner of creatures. Brell then returned home, sealing his portal within a labyrinthine chamber of mystical Living Stone.

And when the other gods came to Norrath, Brell Serilis approached each of them, and after some time convinced them to meet as one to discuss the fate of the world. The Great Mother Tunare, and Prexus, the Oceanlord were in attendance, and Rallos Zek, the warlord, was also there, yet in mistrust kept his distance. Brell, carefully avoiding all queries as to the origins of his information, told of Veeshan's discovery of the new and potentially powerful world in which she had deposited her brood. Words befitting the King of Thieves poured forth from Brell's lips and he proposed that they accept an alliance of sorts, to which all save Rallos Zek agreed, ignorant of the fact that Brell had already released some of his creations into the Underfoot of this new world.

The planet that would be called Norrath was divided up among these beings of power for the purpose of keeping the Wurmqueen in check. Each would create a race of beings to watch over Norrath and keep a vigilant eye on the schemes of dragonkind. Brell claimed the bowels of the planet and created the dwarves, stout and strong, deep beneath the mountains of Norrath. In the abysmal depths of the oceans Prexus left his children, the Kedge, hearty aquatic beings of great mental power and stamina. And on the surface of Norrath did Tunare create the elves, creatures of limitless grace and beauty, and Rallos Zek the giants, fierce and formidable beings, intent upon the defense of their lands. Thus began the Elder Age.

***

Fizzlethorpe Bristlebane and Cazic Thule came next to Norrath, and Brell met them, concocting a second pact with these latter gods, wishing another excuse to create more peoples into the world. Rallos Zek again watched from afar, determined to add to his creation as well, and this time Brell convinced the Warlord to join the pact, assuring him that it was indeed an appropriate time to fulfill his desires.

Deep in the earth did Brell return to create the gnomes, resembling dwarves to some extent, yet more wiry and gnarled, consumed with the tinkering of devices even more so than their cousins. On the surface, away from elves and giants, Bristlebane made the halflings, short and stubby folk, agile and with a propensity to meddle and even pilfer at times. Cazic Thule, Lord of Fear, was drawn to the swamps and jungles of Norrath and there created the green-skinned trolls and reptilian lizardmen. And Rallos Zek returned to the surface, pleased with his sanction to create even more peoples for his army. He made then the orcs, bred for battle and singled-minded in their desire for conquest and the ogres, massive, unmovable beings designed to have all the advantages of any race, hoping that they would develop as superior to all others.