According to the Lego Magazine the Rock Monsters have been tunneling to the surface of the Earth so they can take over the Lego World after mysterious earthquakes from deep in the Earth, and the Power Miners plan to stop them.
A Tremorox from the Summer 2009 sets. They are shown to have some primitive technology, like catapults. The Rock Monsters eat crystals, which give them power. If a rock monster eats a different colored crystal to what it is it will burp and break wind, according to the video released by Lego Club TV on Youtube. In Summer 2009, a new type of larger Rock Monster was released. They appear to be larger versions of the small monsters. The large version of Sulfurix is called Geolix, while the large version of Meltrox is Tremorox. As well as those, another rock monster called the Crystal King or Rock Rage (even look on lego.com) was released in May 2009 as the biggest Rock Monster Yet! He is now awoken form his sleep (Look at Lego.com).