Level 12 is the fourth and the last Caves-style level in the original Diablo, and logically the hardest. The player(s) reaches Level 12 after from a stairway in Level 11. Somewhere in Level 12 there is a stairway to Level 13.
This level often already gives a show of what will be waiting in Hell for there can be monster classes like Black Knight or Succubus. It introduces the player(s) to powerful monsters that resists and/or are immune to the magic types, including some that even start casting Lightning spells constantly at the player(s) and some that throw Firebolt-like balls at the player(s).
The following monsters can be found in Level 12: