Tahal was a Breen who ruled as one of the six primarchs of the Breen Imperium in the 32nd century.
Tahal named her flagship the Tau Ceti, after a viper with a slow-acting venom; this was her favored method of executing her enemies.
Tahal's faction was among those warring for control of the Imperium following the death of the emperor. During the conflict, she once occupied the planet Kellerun to use it as a munitions resupply hub during whatever war she was fighting at the time. Her forces despoiled the planet's environment and decimated the population. When the Kellerun rebelled against her at the Twin Gates, she killed all involved save for Rayner, because she was impressed that he fought "almost like a Breen".
In 3191, the Federation claimed to Primarch Ruhn that they had received a competing offer for Moll and L'ak from Tahal. This was a bluff to deter Ruhn from attempting to seize them by force, as if the USS Discovery delivered L'ak to Tahal it would greatly bolster her claim to the throne. While Ruhn didn't believe it Rayner, now a Commander in Starfleet with a personal hatred for the Breen, used his personal knowledge of Tahal to sell the bluff. President T'Rina offered Ruhn an alternative where the Federation kept custody of Moll and L'ak so that none of the Primarchs could use the Scion, but the negotiations were derailed by L'ak's unexpected death. (DIS: "Erigah")
As the Federation and Moll closed in on the Progenitors' technology, the remaining five Primarchs became aware of Ruhn's death at Moll's hands. Tahal mobilized her fleet to chase after Ruhn's dreadnaught and take the other Primarch's soldiers for herself in order to strengthen her bid for the throne of the Imperium. Detecting Tahal's movements, the Federation was worried about the implications of her learning what everyone was after. Although Admiral Charles Vance mobilized every ship in the sector, most wouldn't make it in time and Tahal ignored President Laira Rillak's attempts to communicate with her. In response, Federation Ambassador Saru volunteered to take a shuttle equipped with the new pathway drive to intercept Tahal's fleet and delay her, possibly forcing Tahal to open communications. (DIS: "Lagrange Point")
Saru and Commander Nhan succeeded in intercepting Tahal's fleet of three Breen dreadnaughts and Saru made contact with Tahal. Saru warned the Primarch that her intervention in a battle between the USS Discovery and Ruhn's dreadnaught could escalate the conflict into a war with the Federation which wouldn't serve Tahal's interests and her most prudent course would be for Tahal to return her attention to the Imperium throne. Tahal was suspisous that Starfleet ships would engage with vastly superior dreadnaughts, particularly near black holes, but Saru brushed off her questions and extended an offer by the Federation to establish trade routes from the Primarch's border through the L'Tar Nebula which would bolster Tahal's claim to the throne.
Although Tahal rejected the offer, she gave Saru all of the information that he needed during the conversation. Saru challenged the Primarch for being too cowardly to take a step towards a future with the Federation. Tahal threatened Saru, but he warned her that Tahal would never claim the throne if she were to kill him. Saru had come as an emissary of the Federation, but he was a Kelpien – a predatory species – and had studied Tahal like prey. Starfleet Intelligence already knew of the Primarch's bases in the L'Tar Nebula and by rejecting their offer, Tahal had confirmed their existence. The planets in that region were preparing to attack the bases unless Saru ordered them to stand down, which would cost many lives but weaken the Primarch's position in the civil war thus costing her the throne. Realizing that Saru was telling the truth, Tahal called off her fleet, but left behind a scout ship to observe matters. However, Discovery was able to use its spore drive to send both the scout ship and the dreadnaught to the galactic barrier where they would need decades to return. (DIS: "Life, Itself")
Tahal was played by Patricia Summersett. She is, to date, the only female Breen to appear onscreen.