Tip of the Iceberg
Ryu was fond of Hecate's corpse refinement method, but he didn't like being restricted to Grave Qi. The only solution in this case was to create his own Death Seal.
This wasn't as difficult as a task as it seemed. After all, Hecate was only of the Path Extinction Realm, a level above the Immortal Ring Realm, yet she forged a new path of Necromancy all on her own.
Of course, if others heard Ryu's thoughts, they'd probably lash out in anger. It might be easy to him who modified Mental Realm techniques on a whim, but not just anyone could do it.
Still, it took Ryu over three years of research to succeed. He had to cross reference many Death Seals and decide what could most appropriately replace Grave Qi, but he eventually succeeded.
The Qi he had decided on was Weeping Willow Qi. It was a Death Qi that was especially fond of slowly corroding high grade Spiritual Herbs. Many descendants of the Immortal Sakura found it to be their archenemy.