Darth Cognus is an Iktotchi assassin, known before her elevation as the Huntress, who becomes the third holder of the Sith mantle under the Rule of Two. She is a creation of Drew Karpyshyn's Legends novel Dynasty of Evil (2009), the closing book of the Darth Bane trilogy; she does not appear in Rule of Two (2007).
Hired to catch him
She is contracted to capture Darth Bane, and does — which by the standards of the trilogy is an extraordinary credential. Having succeeded she concludes that her future runs through him rather than through the fee, and asks to be taken as an apprentice.
The waiting game
When Darth Zannah finally challenges Bane on Ambria, Cognus does not intervene on either side. She pledges herself to whichever of them survives. That is the cleanest statement of the Rule of Two's actual logic anyone in the trilogy makes: the doctrine does not care who wins, only that the loser stops existing.
Iktotchi precognition
Her species is canonically precognitive — the same trait the Jedi Master Saesee Tiin has in the prequels — which Karpyshyn uses to make her a genuinely different kind of threat to Bane's brawling and Zannah's illusions.
Canon status
Star Wars Legends. Cognus exists only in the Bane novels; current canon has never depicted the succession after Bane.
