Suggestion

Suggestion is the ability to project as an inner voice to others, implanting incredibly strong suggestions into their minds, subtly bending them to one's will.

Description
Victims or targets will hear the user's suggestions as an inner voice, thereby causing them to believe that the thoughts and ideas they have are their own, which will in-turn influence their actions in the user's favor. However, as pointed out by Leo Wyatt and demonstrated by Cupids, suggestion does not give the user control over the minds of others. The user may plant thoughts and ideas, but the victim is still in control of their own actions and may resist the suggestion if they become aware of the manipulation.

Suggestion is neutral, it can be used for bad or good. While the forces of good will seek to help others with their suggestions, evil will try to manipulate others for selfish reasons or use it to hurt them in some way. As a matter of fact, the M.O. of certain forces of evil is to drive good beings into taking their own life. Another M.O. of evil is to compel good beings to kill. Suggestion also appears to be a natural part of the invisible form of astral projection. It allows the user to appear invisible and make a telepathic connection with someone, often allowing them to subliminally influence another being by implanting thoughts.

Guardian Angels
Guardian angels are protective beings who protect people in their every day life. They use this to whisper warnings or advice to their charges when they are in danger. For instance, if a man is in danger of being hit by a car because he's about to cross the road without looking, his guardian angel will appears before him and suggests that he stop and look before crossing the road. Without their guardian, charges are left exposed to danger without warning and become very accident prone.

Barbas
Barbas was able to utilize this ability after he learned how to astral project and remain invisible. Together with his power to read fear, he was able to project as an inner voice to his victims, preying on their fears and doubts, and then suggest ways, which would usually end up destroying them, to overcome it. He used this power to attack The Original Charmed Ones and Cole Turner bringing them self-doubt and was even able to trick Paige into stripping Cole of his powers so that he could steal them. Years later, he manipulated Leo into thinking that Zola was evil and Leo proceeded to strike Zola down with lightning and vanquished him before coming to his senses.

In 2033, Barbas attacks once again and forces the Destined Ones to believe their worst fears and doubts. While most were able to overcome there fears, Wyatt was unable to and died believing what Barbas had told him about Prue being dark.

Cupids
Cupids and other beings in possession of a Cupid Ring can use it to slip into people's hearts and minds to implant thoughts, influencing their moods, behaviours and actions. The ring is mainly used to implant feelings of love into a person mind, but in the wrong hands, can be used to suggest reasons to hate as well as to harm others. It is implied that it can also be used to make other people recall past memories. A common practice among cupids is to first slow down time when making love connections, and then project as an inner voice to charges, setting them on the path to love. Leaving their charges unaware of their presence and influence.

List of beings who use(d) Suggestion
Natural Power Through spell, artifact, another power, etc.
 * Guardian angels
 * Spirit Killers
 * Phoenix Witches
 * Siobhan
 * Darla
 * The Source of All Evil  (Through his power of Mind Manipulation)
 * Cupids
 * Coop
 * Prue Halliwell
 * Charlotte Halliwell
 * Cassandra Halliwell
 * Barbas  (By playing on his victim's fears.)
 * Billie Jenkins  (While wearing the Golden Belt of Gaea)
 * Cole Turner  (In the Cosmic Void)