Ervin wrote:That says that so called Mara the demon is karma. Isn't demon a being with intelligence and karma an impersonal physical law?
There are both.
A "māra" can refer to a personal mental obstacle of sorts (kleśa-māra) while it can also refer to an intelligent being. The Māra King is a deva named Paranirmita-vaśavartin or Pāpīyan in the sixth heaven of the desire realm. There are numerous other lesser māras, demons and spirits which Astus outlined above.
These demons are not physical beings as we would normally perceive them. The word "spirit" in English might be best used to describe them. They can directly attack people and this often manifests as mental illness, but can also result in possession or in milder cases otherwise irrational impulses. People undergoing emotional trauma are especially susceptible to such possession. Both Tibetan and Indian Ayurveda diagnose and address these illnesses. Daoism and some schools of East Asian Buddhism are able to address these problems as well. Some western occult traditions like the Order of the Golden Dawn have their methods too.