I would be shocked if I were to run into a fantasy enthusiast – be it tabletop player, reader, or video gamer – who hadn’t heard of Mithral Armor. Sure, maybe they will know it as Mithril Armor or Elven Chain, but they know it. It’s magic armor that’s super light and lets you ignore Strength requirements and Dexterity (Stealth) penalties. Everyone loves it. Well, do you know what else people love? MATH! Ok, not really. Most people hate math, for some reason. Having armor that would allow them to worry about math a little less would probably be pretty stellar for these folks.
Armor (medium or heavy, but not hide), rare
Mathral is a dark, almost airy metal found in the astral plane. A mathral chain shirt or breastplate can be worn under normal clothes. This armor slightly warps the reality of the wearer in small, but noticeable ways. Whenever you make an ability check, attack roll, or saving throw round the result of the 1d20 to the closest number ending in 0 or 5. This does not allow rolls that are not normally critical hits to become critical hits.
Whenever you round down, you may choose to gain Inspiration. You may gain Inspiration this way once without penalty. If you do so more than once, each time you gain Inspiration from this armor your next ability check, attack roll, or saving throw is made with disadvantage. The ability to gain Inspiration without penalty is reset the first time the armor is bathed in starlight each evening.