Dark Magic

The denomination "Dark Magic" to indicate a form of corrupted and evil magic is actually wrong, for magic is basically neuter, not good nor bad. What's important is how magic is used and the motives that influence a sorcerer or a witch to use such magic.

Anyway, there are some applications of magic or some spells whose effects or purposes are undoubtedly evil and can only be used to inflict harm upon someone.

The main ones are:
 * The use of poppets to inflict pain on the person that the doll represents (The Dragon's Call).
 * The creation of an Afanc with the plague this creature's venom produces (The Mark of Nimueh).
 * Necromancy (Excalibur, The Sins of the Father, The Fires of Idirsholas, The Tears of Uther Pendragon).
 * Enchanting a Mandrake Root to traumatize the spell's victim with hallucinations (The Tears of Uther Pendragon).
 * The use of a Phoenix Eye and the binding of a person's likeness to the Magical "gift" (The Eye of the Phoenix).
 * The Cup ritual that can give someone immortality (The Coming of Arthur).
 * The blood-sacrifice performed by the High Priestesses of the Old Religion to release the Dorocha (The Darkest Hour).