However, Andy and Meph drop many other good items in NM, like … Doing Nightmare Runs against Andy and MEPH, …
Runes 17 is the TC that we will be concerned with today, because it is the only Treasure Class that has a chance to drop Zod, the highest rune in the game. Every monster has its own Treasure Class, which determines both the odds of what items drop and the quality of those items. Whenever a monster dies in Diablo 2, the game 'rolls' to see what items are dropped.
The Countess High Rune Farming - Diablo 2.