Morkus from Orkus
That isn't true. We know that 10 minutes in front of the tattoo studying it or practicing it and a successful arcana check will work. And we know that if prisoner 13 tells them an illusion will work, they will know. And I've brought up the Keen Mind feat as a different way to know and I wouldn't even require the arcana check since it's perfect recall. And there might be other ways I haven't thought of. Heck, they're 4th level, so a Detect Thoughts might work.So no matter what creative idea the players come up with to mimic the tattoo, you'll say "No, that doesn't work"? That seems to be the implication here.
However, the party does not KNOW illusion will work, and the party does now KNOW if drawing it will work. That lack of knowledge means that the smart party will stay to negotiate or get KNOWLEDGE a different way, because leaving and having to come back weeks later if their assumption fails will almost surely mean they have no further access to prisoner 13.
If that's the implication, it's a completely false one.