We have turned the Sigil System into a System Reference Document (SRD) under the name Sigil Atelier. You can use the Sigil SRD to build and create your own games, and you can even sell those games by using the SRD’s licence. It’s absolutely free to use, remix and modify it, and there are no hidden costs to selling it. We’ve used the Creative Commons licencing to make it as easy and painless as possible to use.
We have compiled the various bits and bobs of the Sigil Atelier on a Google Drive folder that you can find HERE.
The SRD will be a “living document”, not something set in stone. We will update it as time goes by so that it is always in the best possible state. We have a changelog on the Google Drive folder so you can keep track of what has changed, and the Licencing text and the Sigil Atelier logos are also in there.