It’s RNG my friend. Only Rng
Whenever a monster dies in Diablo 2, the game “rolls” to see what items are dropped. Every monster has its own Treasure Class, which determines both the odds of what items drop and the quality of those items.
Runes 17 is the only Treasure Class that has a chance to drop Zod, the highest rune in the game. If Runes 17 is rolled on an item drop, there is a 1:5171 chance that Zod will drop. If a Zod does not drop, then the game rolls on Runes 16. Runes 16 has a 2:3962 chance for a Cham, and a 3:3962 chance for a Jah. If neither of those runes drop, then the game rolls on Runes 15, and it continues down all the way to Runes 1, which is guaranteed to be an El or Eld.
The only way to roll on Runes 17 is to drop it from a higher treasure class, Act 5 (H) Good . All monsters and chests in Level 85 Areas (as well as Nihlathak) have the ability to roll on this Treasure Class. If A5G is rolled, the odds of Runes 17 are 14/130, or just over 10%. The odds of rolling A5G vary from between 1:150 to 1:5, depending on the monster and how many players are in the game. At the end of the day, maximizing the odds of rolling on the A5G Treasure Class is the best way to get high rune
Just as an example…a cow in hell has a 1:356143 to drop a Ber on player 1.
Keep in mind that if you play a lot you don’t increase this chance. Each run you make the chances to drop are the same, they don’t improve with the numbers of runs you do. The game doesn’t care how much time you invested and does not adjust drops just because you were persistent.
