Ur was a magician of Dalaran before the coming of the Scourge. He was known to have made the best studies of elixirs and for have starting the reasearch to make super soldiers, which he later on named "Worgen". His knowledge was great, but his conscience held him from true power. He died during the Siege of Dalaran at the hands of the Scourge.
The dark mage, Morganth also posesses a book of Ur, Ur's Treatise on Shadow Magic, which the mages of Stormwind want to have returned because they fear that Morganth may find a way to open it and read its contents.