Board Thread:Improvements and Issues/@comment-30535075-20180518203239/@comment-30535075-20180519135149

That's a fanfiction and poor speculation. Yhwach's reiatsu enter Kazui's room through Soul Society. Even if Kazui destroyed Yhwach, that was a tiny portion of his reiatsu. Yhwach's corpse and reiatsu was sealed by Zero Squad as the new Soul King in the novels. Even in the manga, it's said to be remains of Yhwach's reiatsu from the underground debris.