While commenting on this post from Dan about a look at Gygax's notes for Castle Greyhawk, I mentioned that magical armors and shields get even fewer special properties than magical weapons. So, in the spirit of hitting two birds with one stone by following up on that and my previous post alike, here are a few ideas for interesting defensive equipment. These ideas are rougher because I haven't tried to make use of special magical armors in actual play, honestly, but they sure sound more wondrous than just another numerical bonus.
1. The Skins of Siegfried
A piecemeal set of ruddy leather armor, fairly unremarkable save for the distorted face on the back of the torso piece, still sporting a scraggly bit of hair at shoulder-level. This grants AC as leather armor and has a 3-in-6 chance of negating all damage from physical attacks unless the wearer struck from behind.
Wearing the oil-boiled flesh of another person is frowned upon in most societies, of course, and those that don't are likely to have an abundance of would-be thieves.
Inspiration: Siegfried from the movie duology Die Nibelungen (or as I learned of him, Siegfried from the Mystic Circle album "Drachenblut")
2. The Immutable Shield
This round metal shield is emblazoned with arrows radiating out from its central boss like a haphazard star. The wearer becomes immune to magics that would reshape or mutate their body (such as polymorph) or befuddle their mind (such as confusion), gains a +4 bonus to saves against chaotic (or wizardly) magical effects against which the shield can be interposed, and suffers a -4 penalty to saves against lawful (or priestly) magical effects.
Inspiration: the Chaos shield from Michael Moorcock's "Sad Giant's Shield"
3. Trappings of Heroism
These spotless silver pauldrons and their attached flowing cape bring out the best in their wearer. The wearer's Charisma bonus to reaction rolls is doubled (to a minimum bonus of +1), and if they are not wearing any other armor (shields are permitted), they are granted AC equivalent to chain armor. However, any incoming melee attack that strikes has a 1-in-20 chance of landing a telling blow on the wearer's unprotected head, causing a debilitating concussion (if a save vs. death/Fortitude/physical attack/etc. is passed) or instant death (if the save is failed).
Inspiration: the Helmets Are Hardly Heroic trope and massive pauldrons in WH40K armor
4. Infernal Plate
This suit of plate mail looks like it was forged out of rough slag. The gaps and crevices glow dimly with embers when the wearer begins to exert themselves physically. On the wearer's turn in combat, there is a 2-in-6 chance for the diabolical spirit bound within to seep into the wearer's world, dripping with molten hellfire. For the remaining duration of that combat, the wearer becomes resistant to damage from fire/heat, suffers a -2 penalty to saving throws against poison effects, and deals minor fire damage (as a dagger) to anyone striking them with melee attacks.
Inspiration: the Smelter Demon from Dark Souls 2