Who is the antagonist in the novel, The Weirdstone of Brisingamen?

Garner's evildoers are not individually memorable, probably because there are so many of them. Wearing her everyday identity as a local eccentric, Selina Place is vaguely comic. Even as the Morrigan, or arch-witch, she often seems befuddled. Grimnir, who first appears as a terrifying fog, is better, but his fog-identity is so strong that it is difficult to imagine his other guises. Behind these major villains is Nastrond, the Great Spirit of Darkness, who never appears in his own person. He supports the villains with an astonishing array of wicked footsoldiers: svarts, lesser witches, twenty-five foot high green troll women, devil cats, animated scarecrows, and the Brollachan—an empty black presence with red eyes who inhabits living creatures until they burst with the strain of hosting him. At the end of The Weirdstone of Brisingamen the loathsome Grimnir is revealed to be the brother and evil double of Cadellin, a surprising twist, but one that does little to further the plot.