Mining Gulduril Ore gives the achievement "A Sickly Light"
It is about as rare as gold and spawns under y=32. It gives off a sickly green light that is like the walls of Minas Morgul and drops Gulduril when mined.
The current uses for this ore (namely its crystal) are making Morgul Portals, Morgul Torches, and Morgul steel which is used in crafting Morgul blades and armour. Morgul steel is stronger than Orc Steel, but not as strong as Mithril, or even Black Uruk Steel.