Skeletons, ghosts, specters and apparitions are all made of soul power.
Soul power is what motivates them.
Su Ping changed his opinion about undead creatures when he reached such an understanding. He could even transform into any type of undead creature by adjusting his soul power.
That kind of power would be an ultimate treasure for the Little Skeleton.
However, Su Ping didn't intend to summon the Little Skeleton right away. The Abyssal Hole was too much for the Little Skeleton to bear; the little one would probably dissolve immediately, even if it could resurrect.
Su Ping needed to set a "dosage", so that the Little Skeleton could gradually adapt itself to the new environment.
This Abyssal Hole… seems to be alive.
Su Ping seemed to be coordinated with the essential soul power around him somehow, while his soul was condensed.