This is the masterpiece of the Dark Refinery Master. In the early times of the Sacred Empire, there were some dark Demon Spiritualists. It was a mysterious group that would often commit extremely evil acts, like making Soul Puppets.
After making a Soul Puppet, they would seal a deceased expert’s soul inside and then use inscription patterns to control the Soul Puppet, making it work for them. After the founding of the Sacred Empire, the dark Demon Spiritualists were wiped out in a large scale. Things like the blueprint of a Soul Puppet, have mostly been destroyed.
The blueprint the Nie Li found in the Heavenly Marks Family's Treasury is for a bird form Soul Puppet. When he subdues the Thunder God’s Meteorite Sword the spirit of Ye Yan no longer is bound and will soon dissipate. To stop this Nie Li builds the bird form soul puppet according to the blueprint and binds his soul to it.