This can cover you in a false, illusory appearance. This will always fool bystanders, making you beleive you are whatever you look like. Someone who takes a serious interest in you can make an Investigation or Deceit roll against your Deceit, but must make up some stunt that could reveal you disguise.
This is useful to characters who lack the Deceit skill and who like to be able to conceal themselves anyway, but it also extends what you can do with a normal disguise - it is faster, allows a greater variety of forms, and allows you to spend Magic points as if they were Fortune points on Deceit rolls to maintain your disguise.