Underground battles are battles fought in under the ground. Certain items, followers and character traits have effects that specifically say they apply to "underground" battles.
What counts as "underground"?
- All Underway maps (including Underway intercept battles, quest battles etc)
- Siege battles against some Dwarfs/Greenskins settlements (particularly ones in the World's Edge Mountains).
- It is unclear whether certain Skaven settlements, such as Hell Pit, count as underground battles.