This refers to the type of demon that resides in the Sixth Heaven, also called the Heaven of Free Enjoyment of Others’ Emanations.
This type of demon possesses merits and blessings and enjoys the highest heavenly bliss in the Realm of desire, of which our world is but a small part.
They then mistake such happiness and bliss as ultimate, and do not wish anyone to escape their influence.
When a practitioner has attained a fairly high level of cultivation, his mind-light develops and shines up to the realm of the Sixth Heaven.
It is then discovered by the celestial demons, who seek ways to sabotage his cultivation.
Such action can take many forms, threatening or cajoling, or even helping the practitioner attain false samadhi “wisdom” and spiritual power, with the aim of ultimately deceiving him.
These demons take turns watching the practitioner constantly and without interruption, waiting for the opportunity.
If the practitioner has a delusive thought, they pounce on him or steer him toward things contrary to the Way.
The practitioner’s entire lifetime of cultivation is then over, for all practical purposes.