Soul slave!
This was a very special presence. Its meaning was obvious if taken literally. Soul slave was also a special kind of slave!
There was a distinction between a soul slave and giving up one's Soul Seal.
If a martial expert gave up his Soul Seal and became other's slave—unless this Master returned the Soul Seal to him, he could never improve his strength… even till the moment he died.
However, it was different for a soul slave!
Giving up a Soul Seal was that the slave gave a wisp of the root of his soul spirit to the Master, who could destroy this root as he pleased and let the slave die. On the contrary, in the case of a soul slave, the Master would use powerful magic to imprint a wisp of soul spirit—which acted as a mark—on the soul slave's soul spirit. The Master could control the soul slave with their mind and make the soul slave's soul spirit to break down, thus, killing the slave.