Dangers from the Deva Evolution.

In meditation it is literally possible to play with fire. The devas of the mental levels manipulate the latent fires of the system and thus incidentally the latent fires of the inner man. It is woefully possible to be the plaything of their endeavor and to perish at their hands. A truth I speak here; I give not voice to the interesting chimeras of a fanciful brain. Beware of playing with fire.
This transition period is largely responsible for much of the danger. The right type of body for the holding and the handling of the occult force has not yet been built, and in the interim the bodies now in use but spell disaster to the ambitious student. When a man starts out to follow the path of occult meditation, it takes well-nigh fourteen years to rebuild the subtle bodies, and incidentally the physical. All through that period it is not safe to tamper with the unknown for only the very strong refined physical body, the controlled stable and equalized emotional body and the properly striated mental body can enter into the subtler planes and literally work with Fohat, for that is what the occult does. Therefore is the emphasis laid by all wise Teachers everywhere on the Path of Purification, which must precede the Path of Illumination. They lay the emphasis on the building in of spiritual faculty before psychic faculty can be safely permitted; they demand service to the race every day throughout the scope of life before a man may be permitted to manipulate the forces of nature, to dominate the elementals, to cooperate with the devas, and to learn the forms and ceremonies, the mantrams and the keywords, that will bring those forces within the circle of manifestation.

by Alice Ann Bailey, Letters on occult meditation.

More info : Occult Meditation - Letter V - Dangers to be avoided in Meditation Dangers to the Physical Brain

Occult Meditation - Letter V - Dangers to be avoided in Meditation Dangers to the Nervous System

Occult Meditation - Letter V - Dangers to be avoided in Meditation

Dangers to the Sex Organs