In my campaign the Underdark exists, but none of the surface races except Goblins know about it. As it works out, that includes the entire adventuring party. So instead of mapping out an Underdark for my custom continent I wrote this rule for when the surface-dwellers attempt short-range (up to a day or two) navigation underground. It replaces the Make A Perilous Journey roll.
When navigating the Underdark roll + WIS and lose 1 ration. On a 10+ take 3. On a 7-9 take 2. On 6 or less take one.
* You don’t become so lost you cannot return to where you started from
* You are not ambushed
* You find what you are looking for
* You don’t lose an extra ration and day finding your way
* You discover something unexpected
If the party didn’t have a swordmage who can make equipment lantern-bright AND blink over terrain obstacles I would add something like
* You don’t expend 2 Adventuring Gear per party member on torches, climbing gear, and miscellany spelunking expendables.