Ichigo Kurosaki, had spent a long time in Dangai to gain the knowledge how to use the Final Getsuga. He wanted to defend his friends at all cost and he was tired of death. By defeating Aizen there would be peace, but he had to pay the price for the means to accomplish that feat.
Ichigo had explained to Aizen what it takes to fire a Final Getsuga. He himself had to become Mugetsu and loose his Shinigami powers. When Aizen was sliced in half, Ichigo begin to lose his powers and Aizen’s hollow side regenerated him.
If not for Urahara’s Kido, Aizen could had a chance to run, but that was unlikely. The Hogyoku didn’t recognize Aizen as his master…The Hogyoku before it broke recognized Ichigo as his master and granted him his wish to give him his power’s back, but the Hogyoku only could save one part of Ichigos power.
The Hogyoku couldn’t grant the wish the way Ichigo wanted but he could transport him to another world and make Ichigo Shinigami power’s return with time, it would take long however. But the hollow side of Ichigo managed to somehow survive in him, so the Hogyoku turned Ichigo into a Arrancar while transporting him to a new Universe.
Only becouse, it knew what the future holds.
Will he become again a Shinigami with Hollow powers? Or the other way around? Will he again use Mugetsu to save his new friends…And maybe even love?