The stasis box entity was a gestalt creature that was imprisoned in a stasis box before it was released in ancient Egypt by a priest named Horemshep. A being for whom the Fifth Doctor never knew the name, it was comprised of pure mental energy. It could jump from host to host, each time adding the personalities and memories of its new host to its collection.
By the time that it entered into its final conflict with the Doctor in the Atlantean library underneath the Great Sphinx, it had jumped into Peri's mind, but still held sway over thousands of people. By appealing to the individuals the entity had consumed — the most vocal of whom were Peri; Ibram a farmer; and Shemak, Erimem's old teacher — the Doctor and Erimem were able to shut down Peri's mind, as it was unable to process the multiple personalities. The entity then left Peri and passed a fraction of itself into one of the many nearby stray cats. The Doctor then slipped a telepathic inhibitor onto the cat, which then locked that fragment of the entity inside, making it impossible for the entity to find a new host. (AUDIO: The Eye of the Scorpion)
Erimem bonded with the cat and then took it with her when she boarded the TARDIS. She soon named it Antranak after her mentor. (AUDIO: The Church and the Crown) Eventually, the cat saved the planet Talderun by swapping bodies with a being named Shara. Erimem believed the act of heroism was explained by the cat's innate nobility. The Doctor, however, felt that it was the remnant of the stasis box entity which had guided the selfless act. (AUDIO: Nekromanteia)