The Witches of Morva were a trio of immensely powerful witches.
This trio of immensely powerful and knowledgeable witches lived in the Marshes of Morva in the Dark Ages; of the three, Orddu was clever and manipulative, Orgoch quick-tempered and selfish, and Orwen was romantic-minded and scatterbrained. The Witches hoarded powerful magical artifacts, of which they fancied themselves the keepers. Probably the most dangerous such object they held was the Black Cauldron.
The Witches were known throughout the Prydain as a quasi-divine force that even the Fair Folk and the Horned King did not trifle with; perhaps the greatest testament to their power was their ability to bring the dead truly back to life, something they were reluctant to do, but performed on Gurgi in exchange of the Black Cauldron.
Despite their great power, unsavory lair and ghastly appearance, the Witches weren't evil so much as neutral, being willing to help anyone who paid the right price (be they good or evil) but being careless or even downright murderous when dealing with travelers with nothing to give nor buy.
Behind the scenes
The Witches of Morva first appeared in 1985 in The Black Cauldron. They are based on the character of the same name in Lloyd Alexander's Chronicles of Prydain, themselves a variations on the classic figure of the Fates, though the Disney comics universe's Fates are similar, but separate, characters to the Witches.
The Black Cauldron merely adapts the first few books in Alexander's series, and others revealed that the Witches had raised Dallben the Wizard from birth to his teen years, which was the source of much of Dallben's magical knowledge. Whether this is true of the Disney version is unknown.