Hi, I am in the process of building some villains and I wondered if there is any option, feat, class feature, archetype, prestige class, spell effect or anything else that allow to improve or override the save Dc from magical items or artifacts.
I know Arcane Savant can do that with the caster level but I haven't seen an option that work for DC and would work of a wide array of items including artifacts.
On the fly, hardly any. A staff of the master would allow a user to cast one of the spells from the staff and apply Heighten Spell (if the character has taken it as a feat) to it by spending additional charges to increase the spell level (raising the DC).
However, per the rules for Upgrading Items, any spell effect produced by a magic item can be improved to a Heightened version (increasing the spell level and minimum caster ability score to improve the DC, as well as the required caster level) by paying the difference in cost between the "base version" and the "improved version." Of course, that requires both the Heighten Spell metamagic feat and the appropriate item creation feat(s).
Artifacts, unless in a game with Mythic Rules and legendary items, cannot be improved or upgraded without GM-fiat.
The scroll master, a wizard archetype, gains an ability, Improved Scroll Casting, at 10th level that allows them to use their own caster level and DC when casting a spell from a scroll. I house rule that the ability is really an arcane discovery that any wizard can learn.
I don't really recall much about magic items allowing you to change their caster level and DC. You're usually stuck with whatever stats the magic item has.
alchemists can get discoveries that change how potion they drink act.
1 double the duration, and other let him use his caster level for the potion effect.
and there is the brewkeeper prestige class that can make potion-like droughts, that act as potions (so others can use them). but can be made of up any spell level he can cast. he can also apply some effects to them on as he level (like metamagic etc).
all these can be applied offensively as well with the Vaporous potion feat
Are you the GM, if so just give it whatever DC you want and give it a unique name so its not confused with the store bought version.
Player options tend to be less open ended for balance reasons.
Be careful with this. if the PC's kill the bad guy, they get his gear, with it's extra boosts...