Darkest Magick was an enormous leather-bound tome with a metal lock that held it shut.
It contained numerous dark spells and incantations, and was hinted to be actually sentient to a certain extent - when Willow broke the metal lock, the pages flipped as if blown by a wind without her touching them or doing anything. Given how Willow had sought the book out to avenge Tara's injuries and insanity, it was reasonable and logical to deduce that the book had sensed her desire for vengeance, and so directed her to the spells that would be most useful to her.
An example of a potent spell included in the book was the Thick Air spell: a containment spell that invoked numerous deities, such as a Goddess of Death and Destruction, the Queen of the Olympian Deities, and the Titan-God of Time.
Usage and Consequences
As evidenced by how the spells of the book were powerful enough to harm even the Hell-Goddess, Glory, who had until then seemed to be unstoppable, Darkest Magick is potent and dangerous, which was most probably why Giles had it kept in the off-limits area.
Though the spells in Darkest Magick are indisputably powerful, and more or less guaranteed to cause their user's enemy as much pain as it was possible for the person to suffer, there were still consequences for using such dangerous magic: Willow's usage of the spells in the book actually marked the first hint of her turning into a dark witch. Though she was able to cast a number of spells one after another, her eyes were completely black while casting.