The thing is what happened to juha bach exactly before 999 years ago? or 1000 years ago? ...... i'm sure this is connected to what mayuri told yamamoto...... that its his fault that he didn't kill juha bach before....... what is this guy really? or what really happened to him that he needed to regain his heart for 900 years, his intellect for 90 more years and at last his power for 9 years.....

Did yamamoto or the shingamis then sealed his heart, mind and power in separate ways maybe using kidou.... since juha is described as a "sealed king"..... imo it makes sense that parts of him was sealed, and that is why he wasn't active during aizen arcs and fb arcs..... it took him a while to prepare