Soul Seal

From the manga (timelessleaf scanlations Ch 18.1):

Ledgend holds that each soul is said to have a "soul personality" which is called a "soul seal." If a cultivator can control thier soul seal they can control unimaginable power. However, having a soul seal is Rare.

From the novel (NovelMao Ch 73):

It is said that the soul has spirit and spirituality since the beginning of the birth of a person. A "soul mark" or "soul seal" is a symbol of the spirituality of the soul which is hidden in the human soul thus it’s extremely difficult to discover and unlock. For different soul seals, the direction of the soul master’s cultivation is also different.

The manga uses the following images on the forehead to depict a soul seal: