Carrionites are sapient extraterrestrial species native to Rexel IV, the fourth planet in the Rexel binary system; which have perfected the ability to reshape reality using words, a technique in all ways identical to, and indeed identified as being, witchcraft.
Their natural form is similar to a spectral raven with long thin fingers and able to fly through the air without wings. They are able to assume a humanoid appearance, but this requires large amounts of energy. Few individuals actually manage to look like ordinary humans; the remaining ones tend to look horribly grotesque with wrinkled skin, long crooked noses and chins and sharp teeth; very similar to the traditional depictions of witches in Human folklore.
Carrionites have the ability to fly, dematerialize and rematerialize, and communicate at distance using a practice similar to scrying. Their witchcraft is described as a form of science, and their magic is just sufficiently advanced technology; all based on the power of the spoken word, with lexical, poetic and acoustic factors all acting as important variables. The result, however, is indistinguishable from magic. They are also able to use a sample of DNA from a being to build what is essentially a functional voodoo doll.
Carrionites were present at the ancient Dark Times, before the solar system was formed, but were banished to the realm known as the Deep Darkness. Only three of them are known to have escaped, into 1599 Earth, where they influenced the creation of the Globe Theater and William Shakespeare's play Love's Labor's Won, in an attempt to find the perfect set of words and acoustics that could open the gateway and free the rest of their kind. With help from the Doctor and Martha Jones, Shakespeare managed to prevent this and create the monologue spell which could close the gate forever. The three Carrionites ended up imprisoned in a crystal ball which was taken by the Doctor. Presumably, he found a safe way to get rid of it later.