The Diablo 2 Wiki
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Rune Group

From Diablo 2 Wiki

Rune Groups are an internal game classifications analogous to Treasure Classes. They sort runes by value (17 rune groups; 2 in each ascending with Zod alone in 17) and determine which is dropped.

The odds of a given monster dropping a given rune are very complicated to figure. Higher level monsters (and chests) can drop higher level runes (generally speaking). Some monsters, the wraith-types, have up to 4x better odds to drop runes (partially to compensate for the fact that they drop no normal item). The Countess drops runes at a very high rate, and one rune can always be obtained as part of the Hellforge Quest reward.

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