Lucius was talking with the portrait of Salazar Slytherin about the chamber of secrets and the basilisk that was residing there. Salazar explained him that he had left his familiar there, so that he could protect the school from outside attacks and it was one of the biggest arguments with the other founders, because none of them wanted a basilisk in a school full of children, but they didn’t have a choice, since most healing potions that Salazar was creating, were made of basilisk parts and it was his familiar. So, there was no way that Salazar was going to abandon his giant snake.