Difficulty

A monster's difficulty (also known as its MONSTR) is a number from 1 to 57 which represents the game's idea of how tough a monster is. It is based on the monster's level, then adjusted for any special properties the monster has which might make it extra-difficult. The raw values can be seen in monstr.c.

A monster is eligible for random generation when the average of your experience level and your dungeon level is equal its Difficulty; they will be considered too weak for generation when your dungeon level is 7*Difficulty or greater. (For this purpose, once you have the Amulet of Yendor your dungeon level is considered to be that of the Sanctum, and in the endgame to be the depth of the Sanctum plus half your experience level.) The level of your god's appreciation of a sacrificed corpse is also dependent on the corpse's Difficulty.