This special location is not on any map and may only be reached using Poseidon's Permission obtained through the Market from another player or by catching it using Poseidon's Trident at those fishing spots.
Using Poseidon's Permission teleports you to the guild for one hour and includes a Chest, a Campfire and all fishing tool locations. You do receive a bonus to catch rates when using Poseidon's Trident in the guild but Poseidon's Permission is not available to be caught there. When the timer runs out, you will be teleported to Fellin Island outside the Fishing Guild and can use another permission to reenter the Illusion Guild for another hour.
Be careful not to click on the exit sign when moving in the guild or you will end your time early.
You can change worlds while in the guild and will remain inside. If you used a potion before switching, the potion icon will not be visible but your bonus will still be active. Maintenance locks you out of your chest 2 minutes before it happens so switching is an option to deposit and keep fishing.
See Fishing (New) for a full listing of fish catches and other information.