Many magic items (including some weapon and armor runes) have effects with save DCs. For example, a weapon with the frost property rune gets the following critical effect:
On a critical hit, the target is also slowed 1 until the end of your next turn unless it succeeds at a DC 24 Fortitude save.
The frost rune (and some other magic items) has a greater version with a higher DC, and it is possible for a higher-level character to upgrade the frost rune to its greater version, but it is effectively a different item.
Because the save DC is constant, it does not scale against higher level targets, and the item becomes less effective over the course of campaign. Aside from simply replacing the item with a better version, are there ways to increase the save DC on an existing rune or other item?
Best Answer
In short, no
Besides upgrading via crafting, magical items are not meant to scale in power, with the exception of Relics.
Relics: Some extraordinary magic items grow in power along with a character, gaining abilities that add to an adventurer’s legend. These are called relics, and owning one can define a character more than any other magic item could. Many gifts allow for a saving throw or have other abilities that change as the relic goes up in level. The DC for any saving throw called for by a gift is its owner’s class DC or spell DC. The spell attack modifier of a gift is 10 lower than that DC. A relic’s counteract modifier is equal to its owner’s counteract modifier.
Given Relics are explicitly not balanced against the standard treasure rules, the lack of ways to emulate them with Common Magic Items is intentional.