How do you model opposed Thaumaturgy, such as "Scry vs. Anti-Scry"?

Question

How is opposed Thaumaturgy handled? Opposed Evocation is straightforward: treat it as Attack vs. Block, or simply opposed Discipline checks. But Thaumaturgy has fixed target numbers, and no measure of success other than "it didn't blow up in your face."

Example: The villain doesn't want to be scryed, so they cast an anti-scrying spell of complexity X. Can the players cast a scrying spell of complexity X+1 to find them, or can the villain simply not be spied on magically until the players work out some narrative reason why they can pierce the villain's defenses?

Another example: The players have set up wards around their home that prevent the villain from entering. Can the villain simply throw a huge Thaumaturgy spell at the wards to destroy them? If the players are on the ball (but unable to disrupt the spell in person), how should their attempts to bolster the wards or magically disrupt the villain's spell be modeled?

Answer 1

One thing to remember- other than the time scale and the scale of the power involved, Thaumaturgy is pretty much the same as Evocation, and the overall effects and rules are similar.

This is hinted at in several places in the Thaumaturgy rules, i.e. from YW264:

Like evocation, thaumaturgy ultimately directs and releases magical energy, but in a more structured way. It can be used for some of the same basic effects as evocation, but the results are more elaborate and lasting.

So you pretty much use the rules in the same way as you would for Evocation, but the opposing force will be more variable, i.e. in the example you state, the power in the defending ritual is the target number for the opposing one.

Another reference (from YW265)

For a spell that’s equivalent to a contest, you need to put enough shifts of complexity into the spell to make sure that you come out on top against whatever resisting skill the target may use.

And one last reference (from YW265)

Some effects of thaumaturgy seem to be enhanced examples of evocation effects: wards instead of blocks, long-lasting veils to hide a sacred site, magical traps waiting to be sprung, etc. The complexity of these spells usually equals the power you would need to cast the spell as an evocation, but keep in mind that thaumaturgy allows for much greater strength in effect. It’s not uncommon for a wizard to push the complexity up to get a 10-shift ward.

The largest thing to remember is that the difference is scale, and the fact that Thaumaturgy is capable of a wider range of effects because of the possible power level, not really in the rules.

And to directly address your last example, you could do what you say- but it would indeed take time for the villain to gather the power to cast such a spell if the players' home was adequately warded.

Answer 2

I would have to say that the primary difference between the two examples, is that one would probably be a channeling issue and the other discipline for the rolls. Otherwise I would have to agree with your first example - the complexity at creation is the number to be beat to get by it. However, "throwing a huge spell" would usually set off wards because they're always described like tumblers in a lock, not a chair behind the doorknob.