That was necessary to unite the three seas and build a war machine.
Also enables him to march all the damned magic-users into the meat grinder. (Incidentally, drafting so many males and females into the Doomed Ordeal should weaken the genetic viability of the Few).
This way the Cish become returning liberators.
By crushing the Cishurim power structure 20 years ago, TTT and Kellhus can use mythology and conditioned 'last survivor' as needs must.
Finally, if the ordeal fails and Mog walks, the potential of the Psukhe will be the last chance humanity has to turn the tide.