Amulet of guardian spirit
Wearing an amulet of guardian spirit links your HP and MP, causing all damage you take to be split proportionally between your HP and MP pools as per the spirit shield ego.
Tips & Tricks
This amulet is best suited for berserkers and other characters who do not use much (or any) MP. It is usually a bad idea for spellcasters, who will find themselves low on MP exactly when they need it most (i.e., when they're getting pummeled badly). While training Spellcasting specifically for the purpose of giving yourself more MP to fuel your spirit shield is an option, Invocations or Evocations are much better choices for characters that do not intend to cast actual spells often.
When combined with some sort of channeling ability or the Sublimation of Blood spell, it can serve as a limited form of on-demand healing. This is great for mummies, who can channel constantly without feeling hunger, and is even worth considering for spellcasters in lichform. Deep elves and felids, for instance, have low enough HP that they can be one-shotted by Lehudib's Crystal Spear and similar dangers. Losing a large chunk of your MP from a hard hit is still preferable to losing all your HP. Bear in mind that if your HP is low enough, even a full MP pool will still do you no good, as some damage will always get through to your HP.
History
Prior to 0.10, the amulet used only MP to shield the wearer (similar to the Song of Shielding spell in the current version), making it less useful for casters.