locked away is the unholy

this room full of snares

to reach the door tread only

on the perfect squares

In my campaign the story will eventually reach a point where the heroes will (probably) need the help of an undead army that was locked away in an old tomb. Something like this shall be the last step to reach the lich they should ask for help. They will enter the room to the right via a teleportation portal (after they solved a really hard riddle) and they see this.

Does anyone dare to solve the riddle in this room?

Yes exactly: the river in the symbol has a (rather thick) black outline. My Idea was to try and match this outline as close as possible on your other rivers, so it would look like the river from the symbol is part of your own river. at least on first glance, I don't think one would ever get an exact match with this...