Once the tree slipped from Norn's control, the unity she had proclaimed disappeared. Jin-Gitaxias openly rejected her authority, while Phyrexian factions attacked one another as readily as the defenders. Vorinclex fell to a Zhalfirin warrior, and the pressure of the planar exchange tore New Phyrexia's structures apart.
Norn was reduced from untouchable ruler to a wounded survivor crawling toward escape. Karn chose to remain long enough to confront her. Mirrodin had begun as his creation, and the oil that corrupted it had arrived through his unknowing actions. He refused to leave the final responsibility to someone else.
Karn dismantled Norn and then crossed to Zhalfir. With New Phyrexia phased away and its ruler dead, the command carried by the oil went silent. Remaining invaders became inert or disorganised. The war ended not because every trace of Phyrexia vanished, but because the system connecting those traces to a single controlling will had been severed.