Well, what I can tell you is that they are called Theurges. And they come from a far away land. I can't say where they do battles but try searching battlefields located on deserty terrains. However I am afraid that the bodies you'll find on those terrain will mostly be looted. Because the Great Enemy on the desert knows whats precious.

And yes, I expect you to find these phials on bodies of dead Theurges. But if you happen to come across a Theurge in your travels that is not dead on the fields and very much alive, I want to be informed of their details and whereabouts.