Invulnerability

Invulnerability: Grants 5 DR/Magic
 * Price Modifier: +3

Invulnerability can be found on random loot armors, docents, and cloth armors. You may also add this suffix to item through Cannith Crafting.

Creatures start wearing magic (+1 enhancement bonus) weapons around CR 12, and bosses earlier. So in most cases a non-boss monster of CR 11 or lower will deal 5 points less damage when they hit you.