To deceive is their pleasure; to discourage, to baffle, to ruin the hero is their happiness.

Some of them are monsters of terrific aspect who abide in lakes or in desolate rocks, as the terrible tri-formed horse whom Fergus mac Leda conquered and by whom he died.
Naturally, as a link between these supernatural worlds and the natural world, there arose a body of men and women in Irish legend who, by years of study, gained a knowledge of, and power over, the supernatural beings, and used these powers for hurt to the enemies of their kingdom, or for help to their own people.

Some were wise, learned, and statesmanlike, and used their powers for good.

These were the high Druids, and every king had a band of them at his court and in his wars.

They practiced what the Middle Ages called white magic.
Others were wizards, magicians, witches, who, like the children of Cailitin, the foes of Cuchulain, or the three mutilated women whom Maev educated in evil craft to do evil to her foes, or the dread band that deceived Cuchulain into his last ride of death, practised black magic--evil, and the ministers of evil.