Thaumaturgy is one of the forms of magic in Mundus. It is the act of working miracles, classically speaking.
It is the act of trafficking with powerful spirits in order to cause events to happen. The most powerful of these spirits, and those most often trafficked with, are the 7 Gods of Chaos.
The act of Thaumaturgy involves combining certain Runes, which are a combination of words, shapes, sensations, emotions and colors, which represent the 'shape' of certain beings in the Void. When combined in certain patterns, mentally or physically, and coaxed by vocalization, these open a sort of valve to the Spirit Realm, which allows in an energy source known as the Ether, that is the raw stuff of spirit-magic.
Thaumaturges allow this energy to flow into them, and then shape it into magical effects. The danger is that the forces that you are trafficking with, from their distant ethereal tendrils, to the pulsating nucleus of their hearts of chaos, are total anathema to sanity, logic, and rational thought.
In order to perform Thaumaturgy, the Thaumaturge must essentially chant, perfectly, in a language intended to be spoken by beings that exist in seven or eight dimensions, and which closely resemble the feeling of plunging one's hand into molten lead while eating a slice of lutefisk.
And one must do this while driving oneself temporarily insane through the consumption of Ether. And if one were to make a mistake – a single syllable out of place in this chant – one would run the risk of calling something out of that little pinhole-valve into the far-realm, or worse, pulling oneself in through it.
Thaumaturges can do just about anything. Mostly they specialize in the Ether itself, effecting things involving spirits. Enhanced vision, glimpses of the future or of far places, the ability to communicate with beings from other realms, and perhaps summon them to do one's bidding in some capacity. Changing the nature of physical things, or temporarily altering the way the universe functions in a localized sense. But as well, there is the power to infect others with the Ether.
A duel between two Thaumaturges could involve trying to transmute each other into blocks of lead, but more commonly it simply involves building up Ether in a sort of game of magical chicken, waiting for the other guy to either overload and turn himself into a mass of hideous polyps sprouting hair and teeth, or else abandon the attempt and leave himself open to be annihilated in the method of one's choosing, or, most horribly, by waiting until he is at his limit, and then force-feeding him your own power, pushing him over the edge and turning him into something out of I Have no Mouth and I must Scream.
The Ether is easy to build up, but hard to cleanse oneself of, and is prone to pulling more of itself in if not contained, so even when victorious a Thaumaturge must be very careful not to draw too much of the power into himself, lest he start a chain reaction that drags Szadek, Prince of Sodomy out of the spirit world and directly into his small intestine.