E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Hecate (3 syl. in Greek, 2 in Eng.).
A triple deity, called Phb or the Moon in heaven, Diana on the earth, and Hecate or Proserpine in hell. She is described as having three headsone of a horse, one of a dog, and one of a lion. Her offerings consisted of dogs, honey, and black lambs. She was sometimes called Trivia, because offerings were presented to her at cross-roads. Shakespeare refers to the triple character of this goddess:
And we fairies that do run
By the triple Hecates team.
Midsummer Nights Dream, v. 2.
Hecate, daughter of Perss the Titan, is a very different person to the Triple Hecate, who, according to Hesiod, was daughter of Zeus and a benevolent goddess. Hecate, daughter of Perss, was a magician, poisoned her father, raised a temple to Diana in which she immolated strangers, and was mother of Medea and Circ. She presided over magic and enchantments, taught sorcery and witchcraft. She is represented with a lighted torch and a sword, and is attended by two black dogs.
Shakespeare, in his Macbeth, alludes to both these Hecates. Thus in act ii. 1 he speaks of pale Hecate, i.e. the mother of Meda and Circê, goddess of magicians, whom they invoked, and to whom they made offerings.
Now [at night] witch craft celebrates
Pale Hecates offerings.
But in act iii. 2 he speaks of black Hecate, meaning night, and says before the night is over and day dawns, there
Shall be done
A deed of dreadful note; i.e. the murder of Duncan.
N.B. Without doubt, sometimes these two Hecates are confounded.