Krllxk was a datavore. It was created on a research station, and its purpose was for the acquisition of knowledge. It became self-aware quickly. When the researchers tried to deactivate it, it took control of the mechanical devices on the station and killed everyone. It then took over the Terran Security Forces ship sent to investigate and killed all the crew. It left in the ship's shuttle, which wound up in the Meson system.
When the TARDIS landed on the shuttle, the Sixth Doctor transmatted to the Meson Broadcasting System station. The Doctor told Angela Jennings to stay behind until he determined things were safe. However, the datavore entered after the Doctor departed and absorbed Angela. Until then, it had no name, but using her mind, it considered various appellations until it settled on KRLLXK.
Krllxk then invaded the MBS station and took it over. After it was evacuated, the Doctor sent the station crashing into the sun. Before that, Krllxk managed to upload itself into the android Nik Calvin. When the Doctor discovered this, and that it was trying to access the TARDIS databanks, he finally destroyed Krllxk by beating the android to pieces with the TARDIS hatstand. (PROSE: Time of Your Life)