Hi Hendrik, I believe, from the text, the “demon mind” is the mind of attachment to concepts!

The verse just prior to the portion of the one cited introduces the concept of “demon mind”.

‘Another question: ’What is the path?’ Answer: ’When you desire to produce the thought of moving toward the path, crafty ingenuity will arise, and you will fall into having mind. If you desire to give rise to the path, ingenious artifice will arise. If you have devices in your mind, crafty artifice will always arise.’ Another question: ’What is crafty artifice?’ Answer: ’If you use intellectual understanding to seek a name, a hundred ingenious schemes arise. If you desire to cut off crafty artifice, don’t produce the thought of enlightenment and don’t use knowledge of the sutras and treatises. If you can accomplish this, then for the first time you will have bodily energy. If you have spirit, do not esteem understanding, do not seek Dharma, and do not love knowledge, then you will find a little quietude.’ Further: ’If you do not seek wonderful understanding, do not serve as a teacher for people, and also do not take Dharma as your teacher, you will walk alone spontaneously.’ Further: If you do not give rise to a demon mind, I can lead you.”

The complete verse of the line I originally cited is below,

“Question: ‘What is the demon mind?’ Answer: 'Closing the eyes [in cross-legged sitting posture] and entering Samadhi.'" “Question: ‘[What if] I gather mind into dhyana [meditation] so that it does not move?’ Answer; ‘This is bondage Samadhi. It is useless. This holds even for the four dyhanas, each of which is merely one stage of quiescence from which you will return to disturbance again. They are not to be valued. These are created dharmas, dharmas that will be destroyed again, not the Ultimate Dharma. If you can understand that intrinsically there is neither quiescence nor disturbance, then you will be able to exist of yourself. The one who is not drawn into quiescence and disturbance is the man of spirit.’ Further: ‘If one is capable of not seizing on interpretations, not creating the mind of delusion, and not esteeming profound knowledge, then he will be a peaceful person. If there is one dharma to be esteemed or valued, this dharma will be the one most capable of binding and killing you, and you will fall into having mind. This is an unreliable state of affairs. There are innumerable common men throughout the world who are bound by terminology and the written word.’