In addition to your studies of the darkness, you have spent time studying giants and fey.
Prerequisite: Dark knowledge.
Benefit: You can use your dark knowledge on giants and fey. You use Knowledge (nature) for dark knowledge checks regarding these two creature types.
Normal: Without this feat, a character with dark knowledge can only use that class feature on aberrations, elementals, magical beasts, outsiders, and undead.
Source: Heroes of Horror, page 119.