A List of all the lesser-known sorcerers/sorceresses in Arthuriana
A.K.A. Everyone who's not Merlin, Morgan le Fay or Ninianne/Vivien/Nimue. 'Cuz they're oversaturated
Taliesin the Bard (Welsh Mythology)
Ambrosius Aurelianus (Historia Brittonum)
Uther Pendragon (The Welsh Triads)
Menw ap Teirgwaedd (Welsh Mythology)
King Arthur (?) (Welsh Mythology)
The Nine Witches of Ystawingun (Pa Gur)
The Nine Witches of Caer Lloyw (Peredur)
Orddu the Witch (Culhwch and Olwen)
The Nine Sisters of Avalon (sans Morgan) (Vita Merlini)
Queen Ganieda (Vita Merlini)
Queen Guinevere (De Ortu Waluuanii/Enfances Gauvain)
King Melwas (Welsh Mythology)
Sir Tristan (The Welsh Triads)
Iseult the Elder (and possibly Iseult the Fair herself) (Tristan and Isolde)
Eliavres (Perceval, 1st continuation)
The Maiden of the White Hands (Le Bel Inconnu)
Mabon the Enchanter/Mabon ap Modron (Le Bel Inconnu, Prose Tristan, Post Vulgate)
Lady Tryamour (Lanval/Launfal)
Fairy L'Orgueilleux (L'Atre Perilleux)
Queen Morgause of Orkney (L'Atre Perilleux)
King Celidoine (Vulgate Estoire de Graal)
Guinebal (Vulgate Merlin)
Master Elias of Toulouse and his fellow clerks (?) (Vulgate Lancelot)
Vortigern's Clerks (Vulgate Merlin)
Lady Gamille (Vulgate Lancelot)
Lady Brisen (Vulgate Lancelot, Le Morte D'Arthur)
Lady Sebile (Vulgate Lancelot, Le Propheties de Merlin)
The Queen of Northgallis (Le Propheties de Merlin, Le Morte D'Arthur)
The Queen of Sorestan (Vulgate Lancelot)
The Queen of Eastland (Le Morte D'Arthur)
The Queen of the Out Islands (Le Morte D'Arthur)
The Lady of Avalon (Le Propheties de Merlin)
Queen Crisea (Post Vulgate Queste del Saint Graal)
Tanabos the Enchanter (Post Vulgate Queste del Saint Graal)
Darnant the Enchanter (Perceforest)
Hellawes (Le Morte D'Arthur)
Annowre/Elergia (Le Morte D'Arthur, La Tavola Ritonda)
Lady Lynette (and possibly Lyonesse) (Le Morte D'Arthur)
Pulzella Gaia (Pulzella Gaia)
Sir Fellone (La Tavola Ritonda)
The Green Knight (Bredbeddle ver.) (King Arthur and King Cornwall)
King Cornwall (King Arthur and King Cornwall)
Archimago (The Faerie Queene)
Acrasia (The Faerie Queene)
Busirane (The Faerie Queene)
Duessa (The Faerie Queene)
Telorique (Memorial das Proezas da Segunda Tavola Redonda)
This list is tentative and subject to change.
The list discounts Saints and other Christian Holy Persons (such as Galahad). It also discounts characters with "superpowers" (such as Kay and Gawain). In general, the focus is on "Spellcasters"/Magic as an art or skill (Admittedly, the line is very murky).
Fairies (and Goddesses) are included due to the Medieval writers' efforts to rationalize such figures as enchantresses and sorceresses