The New Novel: "We do knot always love You" tells that after Yhwach was defeated by Ichigo, his corpse was still filled with reiatsu and that was enough to hold the universe together. (Implying why nothing happened the first time Ichigo killed Yhwach) so the Royal Guard (To be more precise, Senjumaru's Guards) were assigned to seal his reiatsu and corpse with Hundreds of Seals, to then transport him to where his father once was, the Old Soul King. In the History Books in Soul Society it's written that the Old Soul King was protected, but this is a lied. 10 Years Later, a part of Yhwach's reiatsu was still buried within the rubble, in the final reconstruction area. After the reiatsu resurfaces, the following interation between "it" and Kazui Kurosaki is ambiguous and unknown. But Yhwach's Reiatsu was confirmed to have disappeared in the end.