It is possible using hypnosis with someone will enable an imposter to begin to dialogue with them. This is the greatest risk. It is less a concern with dark spirits, although one might enable one to become more vocal that is inhabiting the person’s energy, so there may be things presumed to come from the subconscious that are supplied by a dark spirit, depending on the energies of the client and the spirit in question, the way the session is conducted, the intention for the session at hand and other energetic variables. The possibility is always present of hearing from consciousness not belonging to the client. The greatest risk to the person is therefore encouraging the possibility of establishing an interconnection with an extraterrestrial imposter. If dark spirits are already present in the energy, talking with them will not necessarily be a huge liability or risk depending on whether the person conducting the session knows how to handle negativity arising within their subject to steer them back to safe ground.
The possibility of creating and fostering an extraterrestrial psychic imposter collaboration will likely have much greater long-term adverse consequences and must be guarded against. A prayer request prior to the session is always advisable. Whether it is successful will depend on the energies and belief quotients of the hypnotist as well as the client in determining whether a foreign consciousness can intrude, just as happens with channelers or with anyone engaging in an intuitive outreach of some kind.