The purple sigil is unlocked via rescuing the ranger and applying the cursed undergarments on yourself. You'll be able to cross the sigil then. Hope this helps, and thanks for playing!
- RegalBuster
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You cannot use the 'enhanced undergarments' you get from the wizard. Only the 'slimy black undergarments' can be used to unlock the purple ring sigil. You will have to go back to an older save.
Are you absolutely certain that you rescued the ranger, applied the 'slimy black underwear' from your Key Items list, and then tried walking over the purple ring sigil? There are no bugs or issues we can see with this scene, so please make sure you are not forgetting something or making an error.