A continuation of a post I made a couple years ago: http://skirmisher.com/node/340 Xorn earth glide like earth elementals but I have them make a subtle change in the soil and stone they pass through. This change draws some creatures, which then draw others.
The first and by far the most common "creature" is the crystal plant. These are living crystals that absorb heat from their surroundings and grow. It is thought that they may be decended from plants that ended up on the elemental plane of earth for many thousands of generations. Though they drain heat and lower the temperature around them, the plants are not usually hazards in of themselves. There would have to be a massive cavern of them to drop the temperature enough to cause damage.
The plants draw two very different creatures. The first is the thoqqua. It eats their crystal bodies and absorbs their stored heat. Xorns generally avoid thoqqua as they are just as vulnerable to melting as normal stone. But occasionally they do work together to kill and eat a small thoqqua infestation. Better to burn a little than deal with the potential lava intrusion later (ala my post on thoqqua). The second creature, the crystal moth, I will describe below.
The thoqqua have their own predators- firebats. These creatures are all but immune to the thoqqua's attacks and drain it of its "blood", something that rarely kills the earth elemental. This means the thoqqua survives and becomes a long term food source for the fire elementals.
Soil and rock on the material plane is full of water, all the way down to the mantle. It may suprise people how much water there is but consider the volume of stone. The altered earth of the xorn draws water weirds- the killer snake version, not the 3e oracles. The weirds seem to find the earth alluring and don't feed on it. They can't feed on xorn as well, so they find cracks in the stone (the same cracks the water travels) and feed on creatures they find in the depths and on the surface. There is only one predator that seems to eat weirds in any numbers, ice paraelementals. Obviously they can not survive in most locations in the underdark, but the paraelementals can find their way in from arctic locations, mountain tops and the densest concentrations of crystal plants. Water wierds can't eat or melt ice and usually can't evade an ice paraelemental for long. Of course most people don't care- an ice paraelemental is just as dangerous as a water weird.
Those creatures are found usually where there is "solid" rock. The rest require miners. These could be dwarves, umber hulks or purple worms. As long as they leave a tunnel behind them, miner form living space for many creatures.
Earth rats are those rodents that have mutated and become partial elementals because of how long they have spent in the depths*. They have bones made of stone and this makes them prime food for the xorn. Crystal moths are elemental insects that have caterpillars that feed on the crystal plants. Their silk is infused with earth energy** and is worth thousands of gp per cocoon. The moths are in turn preyed on by crystal spiders (not the Dark Sun monster). Their silk is also powerful and valuable but for different reasons***.
Shad are small humanoids from the 3rd Planescape Monstrous Compendium. They are food for many creatures on the elemental plane of earth and so escaping to the prime material plane is something most of them desire. They are drawn to the xorn's altered stone and carve it into tools and weapons. They prefer it to other minerals because of its "sense of home". These creatures could be used in a manner similar to the mythological kobold- mostly harmless fairies that make noises in mines.
And then there is the most dangerous creature drawn to xorn- the chaggrin. This is the earth grue, a predator with a an extremely powerful ability, one that can doom parties. Any earth magic spell within 40' of them is dispelled automatically (and the other grues do that to their own elemental magic). Chaggrin use this ability to attack pech, xorn, sandlings and other earth elemental creatures- stopping their abilities and making them much more vulnerable than usual. Fortunately for the xorn, this has no effect on earth glide (though I might change that in my settings). In either case, chaggrin follow altered earth passages like bloodhounds scenting for food.
* Before you ask, yes I would consider making duergar, drow and monsters who stay in the depths for their entire lives partially elementals. Only those who visit the surface or live near a massive amount of non-earth (a lake, volcano or gate to air) can avoid this fate.
** This would be the radiation mentioned in the D series of 1e modules. It prevents teleportation, makes magical weapons and armor more powerful and may have other uses if the GM wishes.
*** It stops earth gliding and can be easily hidden on stone. People trying to keep earth elementals at bay will pay good money for this silk. Keeps them from coming up through the earth below.
