Conversation so far:
what was imperial in regard to cassandra nova ?
Imperial Guard.
what was the imperial guard for?
Cassandra also made Lilandra send the Shi'ar Imperial Guard to wipe out the mutant population of Earth, starting with the X-Men.
was this successful?
The Guard fought the X-Men until they were able to show the truth to the Imperial Guard.

Question: what was her greatest accomplishment?
Self-contained rewrite: What was Cassandra Nova greatest accomplishment?