Chapter 164
Unlike regular spirits, anti-spirits are tethered to his will, making them extensions of his power.
Which basically means that unlike regular spirits, which might have their own consciousness or require a contract, anti-spirits are extensions of the summoner's power and obey his commands without question. They are bound to his will and cannot act independently.
This connection ensured that the summoner has complete control over their actions and abilities.
Just as there is matter in the universe, with its opposing force being anti-matter, there are spirits, ethereal beings of light and energy, countered by anti-spirits, dark entities born from the Abyss. While spirits embody harmony and life, anti-spirits are manifestations of chaos and death.