Source: Kundalini Teacher
By an entity, we are referring here to energy based beings that take up residence within people. Most entities are created by humans, usually unconsciously. Thoughts are things, they are energy and energy and matter are one. E=MC2. So any thought that is fed emotional energy gets bigger and more powerful. What you focus on, will grow: this is the power of free will, and the essence of all prayer, spell casting and the root of much mental illness.
You nurture whatever you choose to give your attention to. Mindfulness, is paying attention to your thoughts and being choosy about which ones you feed. Anything that you give enough attention to, can potentially “wake up” and become self aware.
Thoughts in the body do seem to take on a life of their own, eh? Remember how your toys as a child seemed to become real beings? Your love gave them consciousness. Ritual objects do this a lot, so cleanse and rededicate them regularly.
Usually with the creation of entities, the focus will be a fear of some kind, because fear is separation and an entity experiences separation from the Light. It is separate from you, a parasite. When a fear based entity has been created, like all sentient beings it will act with self preservation and attempt to propagate. Feed and breed. In this way, humans have created uncounted millions of diverse beings.
These beings range from the not very aware stray thought forms left over from another lifetime, to very powerful Old Gods and Archetypes, like the Greek or Egyptian pantheons that have been fed and shaped by centuries of worship and love. The difference being, the Old Gods are not separate from the light and generally are more helpful than not.