Bloodcap Mushrooms

The Bloodcap Mushroom is an edible species of fungus that is prevalent in the Underdark and the Shadowfel, making it a valuable resource in both environments.  The fungus grows to maturity quickly and can be preserved fairly easily, though it will rapidly wilt into a dark slimy substance if exposed to direct sunlight.
The fungus gets it's name from its growth pattern: bloodcaps grow in fresh or congealed blood, usually that of a deceased body or the site of a battle.  It is even possible for them to grow on the bodies of some undead creatures, assuming that they have blood and are not exposed to sunlight.  The mushrooms grow to between 1 to 2 inches tall and roughly 1 inch in diameter, with thin but sturdy stems.  They have a dark, washed out red color and a rough texture.
When eaten they are an excellent source of iron and protein, but have a metallic taste that is, unsurprisingly, similar to blood or liver.  This flavor can be mitigated by cooking the mushrooms, and when boiled in water they can be the base of a savory broth with greatly reduced unpleasant metallic flavor.
Bloodcaps are a rare food source in the the Underdark, but are of particular value in the Shadowfel, where little grows that is edible to visitors from the material plane.  These mushrooms in the shadowfel grow commonly in graveyards, on undead and in the dens of some monsters.  Because of the lack of sunlight in the shadowfel, these mushrooms almost never deteriorate naturally.  However, they are of interest to herbivores and carnivores alike, as they are digestible by both.
Some carnivorous species will even maintain patches of these and hunt the animals that come to eat them.  Intelligent creatures in both biomes will also take advantage of opportunities to grow them and some will even harvest blood from bodies to grow them intentionally.