Tragic news! The big bad has put up a "DNI if you're an adventuring party or on a quest to stop evil", so it looks like there's nothing the PCs can do about their scheme :(
Get the illiterate Barbarian in here!
I don't have DNI's but there are certain rules.
-> Evil Lair is closed to all visitors on Wednesdays, Fridays, Saturdays and Sundays unless an appointment is made beforehand.
-> Leave your horses at the designated parking space.
-> Male adventurers are welcome on Mondays. Please approach the western entrance with huge open space in front of it. Please ignore the smoldering remains of previous visitors
-> Female adventurers are welcome on Tuesdays and Thursdays - please come directly to the audience/throne room.
They say bad ideas.
Hence why I immediately believe this is in D&D 5E/5.5E.