|The Mark from Dishonored|
The base difficulty comes from the scope of the Sigil. The scope is essentially what kind of aspect the Sigil is - boost, situation, character, High Concept or even game aspect. It's also possible to use Sigils to grant stunts. Once the scope is determined, the exact aspect or stunt that the Sigil represents has to be determined. From there, how has to be decided - does it add an aspect to the thing that bears it, does it modify or replace an existing aspect, or does it remove or nullify an aspect? These base actions can be summed up simply as addition, alteration and removal (replacement is actually removal and addition combined). There will be some other factors that go toward the difficulty, mainly the quality of the Sigil itself. The more permanent or well-crafted the Sigil, the more potent it has the possibility of being - a Sigil scrawled in charcoal on a piece of paper isn't going to hold up as well as one carved into stone and inlaid with silver. Sigils can be created from nearly anything that can be used to make a pattern, from a bundle of shaped sticks to a tattoo to an engraved amulet to architecture. A well-crafted Sigil is one of the most effective ways to increase the Sigil's Resonance.
|Prototype Addition Sigil - this would have to be modified based on the exact effect|
Resonance is a measure of how the Sigil reacts with other aspects - namely those of the person utilizing it. For Sigils that operate somewhat independently of a character - say, a ward on a door - Resonance applies to that object. The base Resonance is determined by the scope of the Sigil, and can be modified by both how well the Sigil is created and how complex the user is aspect-wise. There are techniques for increasing Resonance, and a failed Lore roll can be turned into a success by decreasing it. Whenever the Sigil is activated, the character (or GM, if the Sigil is static) makes a Resonance roll. Failure means the Sigil works, but there is a side effect. I'm still deciding exactly how that works, but most likely it would be a choice between a compel on an aspect or one of the character's aspects altering the effect of the Sigil in an unexpected way.
That sums up the basics of what I have so far, and I'm definitely a lot closer to a workable system with a few tweaks.