Source: Ultimate Wilderness, pg(s). 195
Dweomer caps are a species of fungi that can subsist on decaying organic material, but must absorb trace magical energies in order to reproduce. Because of this, their presence often indicates residual magical activity, or nearby magical treasures or traps.
A typical dweomer cap is a gilled black mushroom about one foot wide covered in wavy purple and orange patterns, and it weighs around two pounds. They are naturally luminescent and change their brightness, shimmering pattern, and hue when in the presence of magical auras, an ability that inspires some spellcasters to adopt them as familiars; indeed, dweomer caps can suppress and shine their light on command when serving as a familiar.