Cassandra was the outermost body of the Agamemnon system.
It appeared to be a comet, but was in reality a TARDIS damaged during the war with the Great Vampires. After the crew was killed and it had not been used for ten thousand years, it shut down power. Drifting near Agamemnon, if fell into an erratic orbit for the next ten million years. It sent out a distress signal, which activated any dormant N-Forms which the signal crossed.
When the alternate Seventh Doctor created by the reality Nexus on Iphigenia and merged with Emil Zatopek learned about it, he had Roz Forrester take him to Cassandra so he could destroy it before the Time Lord technology could fall into the wrong hands. (PROSE: So Vile a Sin)