The Phyrexians invaded the Sejannese Toral Pugnapax after their Omniscientists made forbidden deals with them. The Phyrexians provided machines to the Omniscientists while the Omniscientists provided them with magic items. However, in time, the Phyrexians’ overlord and demigod, Yawgmoth, gave the command to invade Sejanna and they did so in record time. In a brief three months, all of Sejanna was overthrown. The capital of Iv Naresis was renamed Primus-One.
Though this three month period was the focus of the Phyrexian Purge, it lasted another ten years.
The Heroes of Crucibal ended the Purge by using the Staff of Law.