Random Encounters in the Underdark
The Underdark is a vast region encompassing more square miles than can easily be calculated, especially considering the three-dimensional nature of it. Outside "settled" caverns and vaults, the dark rootways below the mountains and plains of Faerûn constitute a wilderness.
Refer to the table below to find the percent chance for a wilderness encounter in the Underdark. The first number in each set is for a single hour of travel in that type of Underdark terrain; the second is the chance of a single encounter in 8 hours of travel in that terrain. If the party does not change terrain categories in the course of a day's travel, it's easiest to roll a single check for the day and then determine randomly at what point in the day's travel the encounter takes place, if one is indicated.
|Terrain||Normal Travel||Open Camping/|
|Desolate||5% (33%)||2%(15%)||1% (8%)|
|Wilderness||8% (49%)||4% (28%)||2% (15%)|
|Frontier||10% (57%)||5% (33%)||2% (15%)|
|Settled||12% (64%)||6% (40%)||3% (20%)|
For parties that are moving with unusual caution (one-half the members' best possible speed or slower), use the Cautious Travel category. For parties that are stopped or camped, use the Open Camping encounter chance. For parties that are camping in secret or hiding, use the last column of the table. A party camping with a fire is never camping in secret unless its members can somehow hide the firelight.
Desolate areas are wild caves and tunnels that are not settled and generally do not support much in the way of wildlife or monsters. Much of the Lowerdark is desolate.
Wilderness areas are wild caves and tunnels that are not settled but still support significant wildlife or monsters.
Frontier areas are lightly settled tunnels or caves, but communities are small and far apart, usually separated by miles of tunnels.
Settled areas feature colossal caverns that can be miles on a side, a fair number of small villages and vaults no more than a day's travel from each other, and some tunnel patrols from nearby cities or fortresses (which often enough are drow, illithid thralls, duergar, or derro).
For the purpose of generating encounters, the Underdark has been divided into six encounter areas. The encounter areas given here are: Upperdark (Underground, Settled), Upperdark (Underground, Frontier), Middledark (Underground, Settled), Middledark (Underground, Wilderness), Lowerdark (Underground, Wilderness), and Lowerdark (Underground, Desolate).