From the publishers website:
A free 11-page supplement for Underdark campaigns by Monte Cook
Far below the surface world lie the veins through which the earth’s own blood flows. Earthblood makes possible life in the Underdark. Its mystic energies sustain many of the strange plants and fungi that grow there in the same way that the sun gives life to all flora of the sunlit world. These energies also provide a new spectrum of abilities for characters in campaigns that draw upon the rules of the Open Game License.
This free supplement provides all the information you need to introduce the concept of earthblood into your campaign, including background details, new feats, distinctive spells, specialized items, and DM-only rules.
Ties in to the rules from the Books of Experimental Might! For characters of all levels.