From Lexicanum

The Cardinal Palace was built by Cardinal Bucharis during his Plague of Unbelief, on Gathalamor. It was this initial construction project that drove him to conquer other planets, beginning his empire. It is said to be one of the largets palaces in the Imperium, only surpassed by those on Terra and Ophelia VII.

Sources

• Codex: Sisters of Battle (2nd Edition)