The most important thing for any sadhana or worship

The challenges in Sadhna,

A lot of people consider my explanation as foolish when I explain to take precautions during worship and prayers.

Deity worship requires utmost precautions.

The chanting of Keelit mantras, breaking the vows during worship, not following the rules of practice including Sankalp, Viniyog, Shadnyaas, Kullika, Panchopchar or Shodoupchar, Utkeelan, Mala Sanskar, Aasan purity, Pranayam, or poor maintenance of ashauch kriyas results in inflictions from demonic species such as the BrahmarākŞhasa and powerful retinue of the Pradhan deity. These demonic species take away the bhoga of the Prayers and in turn become very powerful.
They camp around the sadhak’s house and his life. Such people have some kshudrak powers of seeing the future, or predictions and premonitions but the person remains unhappy in personal life. You will be surprised that Granthas like Prashnamarg and several Ayurvedic texts define brilliance as Vyadhi or disease. This kshudrak possession also makes a person seems brilliant. Such people are afflicted by altered ego, unreasonable anger, several unknown diseases, morbidity, and venom for others. Even worshipping the deities inside the house without precautions can lead to these defects.
The sthapathya texts do not mention the Devalaya Vedha without reasons. The defects in sthapathya resulting from Sutak demon, woman problems, ashauch, etc. makes a person a slave of these inferior species. A temple built outside is safe from such problems. However, the problem is that every temple is not a temple. The commercial properties built near colonies and houses are not temples. They are just structures and idols without any positive energy. During Ashtamangalya prashnam, it is discovered that some deities leave their idols and some negative power takes possession of the idol. The worshipping of such idols results in obligation for the soul. The soul takes a the form of a ghost after death in such cases, so be careful when you go to every temple. Sthapatya texts also mention the bad effects of worshipping an unproportioned idol.

The problems with mantra

(We are trying to understand by taking an example from the life of one person that this can also be possible.)

This event took place when I was 20-21 years old. I use to keep fast during Navratras and chanted the devi mantra on daily basis. During one of the Navratras, I was trying to complete the same process. It was a complete fast and nothing happened during the first few days of the Sadhna. However, on the seventh day something very strange happened at night. During midnight, I felt that someone was choking my throat and I could feel the fingers and grip on my throat. So I opened my eyes with horror and saw a demonic figure over there. He was four to five feet, carried a large stomach and long hairs and arms. He had big white tooth and was staring at me. I started remembering the Devi immediately and then he disappeared. I thought that I had some nightmare so took it as some imagination of my mind.
However, the creature again appeared on the second day. This time he was just standing and laughing at me. I stood up to take action but he disappeared. This time there was no illusion, I was pretty sure that I saw him standing over there. So I went to a famous devi worshipper of our town. I narrated the incident to him but he said that nothing like that ever happens. He advised me to wear a rudraksha, which I bought immediately and wore. This was the ninth day of the worship and despite of complete fasting, there was no urge for food. However, despite of all precautions the demon appeared again and this time he was sitting on my chest and choking my throat. With all my courage and strength, I pushed him back and put my hand on his throat and stood up to take action. When I opened my eyes, I found myself standing near the wall. There was no chance that I was hallucinating, I was standing.