You probably don't know who Ur was, and even if you do, you don't know much about him because there's not much to know. We know the following - Ur was a mage who lived in Dalaran before the coming of the Scourge to the Eastern Kingdoms, who apparently died during the invasion Arthas led to retrieve the Book of Medivh so that Kel'Thuzad could use it to summon Archimonde the Defiler. It's possible that Ur, being a remarkably skilled and knowledgeable wizard, was one of the three archmages slain by Arthas while maintaining auras that protected Dalaran from the undead.
What's interesting about Ur isn't what he himself actually did, however, but what was done with the work he left behind. Ur was the author of two tomes (that we know of), The Book of Ur and Ur's Treatise on Shadow Magic. Ur's knowledge of other planes of existence was significant, if faulty - while in practical terms his understanding was great enough that it was possible for Archmage Arugal of Silverpine to use the Book of Ur to summon worgen forth from their prison in the Emerald Dream, it's notable that Ur didn't actually understand what the worgen really were, or what druid magic was - he simply saw the worgen as monsters from another world. Still, without Ur, it is unlikely that Arugal would have been able to bring forth the worgen he did.
Amazingly, despite knowing nothing of druidism and only having a hazy understanding of the worgen origin Ur's research allowed Arugal to pull Alpha Prime and other ancient members of the Druids of the Scythe to Azeroth. Considering it was Archdruid Malfurion Stormrage who placed Ralaar Fangfire and the other druids who'd helped create the Scythe of Elune under Daral'nir (the great tree in the Emerald Dream for which Tal'Doren is a mirror) in the first place, the fact that Arugal managed to draw them forth is astonishing. Yet manage it he did, with Ur's book.
To beard the wolf in its lair
What little we know about Ur is pretty interesting to consider. He appears to have been a master of various esoteric and little studied forms of magic. The Book of Ur contained the secrets that Arugal used to pull the ancient worgen back into Azeroth, putting it on par with the Scythe of Elune (a major artifact created by using the fang of the wolf ancient Goldrinn himself) and his Treatise on Shadow Magic was mage locked to prevent others from reading it, making Ur an acknowledged expert on those magics on par with figures like Gul'dan or Kel'Thuzad. Indeed, considering Ur's presence in Dalaran, it's possible Kel'Thuzad studied with or even under him. But one of the tantalizing hints we get into Ur's personality is the idea that his knowledge was great but his conscience held him back. Since we're told that just reading Ur's Treatise on Shadow Magic could be enough to harm the sanity of an insufficiently trained mage, we're forced to wonder exactly what is in the books Ur wrote, and more importantly, why is it that Ur himself managed to remain a wise and dutiful mage when at least two other mages (Archmage Arugal and Morganth) succumbed to the lure of the dangerous secrets Ur learned and compiled, but never fell to himself.
And of course that's assuming that Kel'Thuzad didn't study Ur's writings, which considering his hunger for magical knowledge (especially forbidden magics) seems unlikely. Outside of the Book of Medivh, the Book of Ur is probably the most complete work on extraplanar entities that would have existed in Dalaran at the time.
To learn but not to act
Just considering what happened to Arugal makes us wonder all the more. Escaping the fall of Dalaran to the Scourge and Archimonde's destruction of the city, Arugal ended up in Silverpine where he used the magic of the Book of Ur at the command of his king, Genn Greymane, and in so doing created the modern worgen curse as well as accidentally butchering Baron Silverlaine and his whole family when Alpha Prime and the other worgen proved impossible to control. Soon, it was Alpha Prime that Arugal answered to, and the poor broken mage created a Wolf Cult to serve the ancient worgen's goal to use Gilneas as a breeding ground for a worgen army in order to find the Scythe of Elune and eventually get revenge on Malfurion Stormrage. Arugal spent the rest of his life surrounded by the very worgen that had turned on him and slaughtered everyone around him, devoted to a cult worshipping the very beings he'd dragged back into the world. Did Ur understand the dangers better than Arugal? Was it conscience or prudence that kept Ur from using the knowledge he'd attained?
And this leads us to the really big unanswered question about Ur - how, exactly, did he attain this knowledge? Medivh, as we've seen previously, gained much of his knowledge of demons and the Burning Legion from his possession by Sargeras, but he still had to make deals and interrogate various lesser demons. If Ur's conscience kept him from similar bargains, then how did he learn so much? Did he simply scry the cosmos? Make contact with a sleeping Druid of the Scythe under Tal'Doren? Considering Ur's knowledge of shadow magics that seems unlikely - even druids fallen to the animal rage of Goldrinn weren't likely to learn shadow magic. We might well suspect the satyrs - not only were they ancient enemies of the worgen-cursed druids, but they are descended from highborne mages and could well have learned all sorts of dark magics from their time as servants to the Burning Legion. But there are no satyrs anywhere to be found in the Eastern Kingdoms - how would Ur have contacted or been contacted by one? And what could he have offered it to coax it to part with such potent secrets? For that matter, could any satyr possibly be powerful enough to undo Malfurion's work and free the worgen from Daral'nir?
Perhaps taught, perhaps simply observed
The prime suspect among the satyrs would have to be Xavius, who at that time was trapped as the Nightmare Lore in the Emerald Dream itself. Xavius would have had the ability to communicate with Ur via dreams, planting the knowledge of how to bridge the Emerald Dream and the waking world (as was shown in Xavius' attempt to merge the corrupted Emerald Dream and the mortal plane, he possessed such secrets) in an attempt to use Ur the way he would ultimately use Fandral Staghelm. The question then would become why didn't it work? Why didn't Ur use what he'd learned? It's rare indeed for a mage, even a noble hearted one, to shy away from using power. This implies something very interesting, namely, that no one was supplying Ur with the information he wrote in the Book of Ur and Treatise on Shadow Magic, that he worked it out for himself without using demonic short cuts and then simply had the self-control and restraint not to use what he had learned.
This makes Ur, if true, one of the greatest mages in the history of Azeroth, because he very clearly had enough power at his fingertips to wreck the world, worked out entirely on his own, and he never once made use of it. I'm still wondering what's in that Treatise on Shadow Magic. How corrupt could it possibly be that the mages worry about what will happen if someone just reads it? Have they read it, or are they just basing this on other people who have? If so, who? We've never seen Ur. He's never appeared in any Warcraft game. All we know about him is that he wrote two books that are apparently so dangerous no one should read or use them, and never once did himself.
I'm not saying that there's a story in here somewhere. Oh, wait, no. That's exactly what I'm saying.
While you don't need to have played the previous Warcraft games to enjoy World of Warcraft, a little history goes a long way toward making the game a lot more fun. Dig into even more of the lore and history behind the World of Warcraft in WoW Insider's Guide to Warcraft Lore.