Mother Talzin is a Dathomirian shaman and Clan Mother of the Nightsisters, introduced in "Nightsisters", the twelfth episode of the third season of Star Wars: The Clone Wars, first broadcast on 7 January 2011. She was voiced by Barbara Goodson. She is the mother of Maul and of Savage Opress.
A second dark side
Her importance to the setting is structural. Before Talzin, the dark side in Star Wars was the Sith and people the Sith had recruited. The Nightsisters are a separate, older, matriarchal tradition with its own vocabulary — magick, ichor, the Chant of Resurrection — that predates the Sith on Dathomir and owes them nothing. Palpatine does not command Talzin. He took her son.
Design history
Her look comes from Iain McCaig's discarded concept art for the villain of The Phantom Menace, the designs that eventually became Darth Maul. That is a neat piece of production history and it explains why she reads as a sibling of Maul's visual language rather than a derivative of it.
Her actual grievance
The Nightsister arc is a revenge story with a legitimate cause. Sidious took Maul as a child, Dooku disposes of Ventress on Sidious's instruction, and Talzin's response is to build Savage Opress as a weapon aimed at both of them. Everything she does across three seasons follows from a wrong done to her clan by the Sith, which puts her in the odd position of being a dark side villain whose enemies are the other dark side villains.
The massacre
The Separatist assault on Dathomir in "Massacre" (2012) effectively ends the clan. It is one of the bleakest half-hours the series produced, and its consequences run forward for a decade of storytelling — into Star Wars Jedi: Fallen Order and the survivor Merrin, and into the canon material that has kept returning to Dathomir ever since.
Canon and Legends
Talzin as written is canon, from The Clone Wars. The Nightsisters as a concept, however, come from the Legends shelf — Dave Wolverton's The Courtship of Princess Leia (1994) introduced the witches of Dathomir, and the Legends version differs substantially in organisation and in its relationship to the Force. Where the two shelves disagree, this dossier follows canon.
Where to watch
The Clone Wars season three, "Nightsisters" through "Witches of the Mist", then season four's "Massacre". The Son of Dathomir comic completes her arc.