Antranak was the name Erimem gave to the stray cat she brought aboard the Fifth Doctor's TARDIS at the end of BFA: The Eye of the Scorpion. Erimem had adopted the cat, who had in the course of events absorbed a malevolent alien intelligence harmlessly into its mind, and named him Antranak after her mentor in Egypt.
Erimem and Peri were quite fond of the cat, the Doctor less so. Antranak quickly made himself at home in the TARDIS, much to the Doctor's chagrin thanks to the cat's habit of jumping onto the console, and refusal to use the litterbox.
Antranak departed in BFA: Nekromanteia. An energy convertor had been built on the planet Talderun to support its creator, Shara, in a heaven-like state of non-existence in a pocket universe while his body anchored the reaction on the material plane. Due to the theft and destruction of Shara's corpse, the reaction was becoming critically imbalanced, and would soon destroy the planet, and ultimately the entire star system.
The reaction could only be rebalanced by another living being mounting an altar and swapping places with Shara, allowing Shara to return to the physical world and die while the sacrificed being became trapped in Shara's pocket Universe. The Doctor prepared to sacrifice himself thusly to save the planet, but Erimem argued that she should be the one to make the sacrifice. While they argued, Antranak leaped onto the altar himself, swapping places with Shara and stabilising the reaction, saving the planet and allowing Shara to die in peace.
The Doctor later wondered if the alien creature in the cat's mind forced him to act. Erimem, however, remained firm in her belief that her noble cat sacrificed himself.