The banshee level is a special level at around D22-D24 (between the Water Temple and the wall of flames). The level can be immediately recognized by a message referring to sorrow and death, and a great amount of corpses located all over the dungeon floor.
Lots of corpses, which might be of some interest to provide missing intrinsics.
Food items do not rot on this level, but the 'rot counter' keeps on going — thus, all the corpses/food items may decompose rapidly upon leaving the level. This can be counteracted by blessing valuable corpses (for example orb guardians) just before leaving the level.
Banshee (consult corresponding page for dealing with her) straight on the downstairs in the middle of level. Room with her should be behind hidden doors.
None, but character must pass this level to gain access to lower part of Caverns of Chaos.