For reasons not yet well understood, placing parts from more than one magical creature (being or beast) inside the core of a single wand seems to cause a conflict that significantly weakens the degree to which the wand strengthens magic spells. This weakening occurs under almost all situations:
Different Elementals - When part of a magical creature having one elemental (e.g., a mermaid hair - Water) is enclosed with part of a magical creature having another elemental (e.g., a banshee hair - Earth)
Different Phases - When part of a magical creature having one phase (e.g., a sliver of unicorn horn - Light) is enclosed with part of a magical creature having the other phase (e.g., a sliver of dragon scale - Darkness)
Being and Beast - When part of a magical being (e.g., a werewolf hair) is
