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 a risky choice 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, it can serve as a limited form of on-demand healing. This is great for mummies or lichform casters, who can channel constantly without feeling hunger, and is worth considering for spellcasters in chunk-rich areas. 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.
Remember to re-assess the priority for killing magic point draining monsters (such as the eye of draining) while wearing this amulet, as their ability to drain your MP is often rapid and irresistible.
History
Prior to 0.10, the amulet used only MP to shield the wearer, making it less useful for casters.