The topic of manipulation, which we all do, is obviously relevant when dealing with others that exhibit traits from the dark triad, and I attempt here to define it by carving out the more malignant form of it.
Emotional manipulation is emotional abuse. A person who controls your feelings and behavior with manipulation does not value or respect you or care about your well-being. 
Manipulation can be difficult to capture in one definition because the essence, or defining feature, depends on which aspect of the concept you want to emphasize. For example, manipulation, which is in the most general sense interfering to get a result, can be hidden in nature in an effort to get what you want, but it can also be obvious and benign. And it can be persuasive in nature by appealing to your emotions or reasoning, which can be either obvious or hidden. This is clearly a multi-faceted concept that can be viewed on different dimensions.
In an attempt to circumscribe the facets of manipulation, there are four dimensions that will help us: deceptiveness (degree to which someone misleads), callousness (degree to which your well-being is not taken into account), sadism (degree to which which one wants to harm you and relishes in it), or covertness (degree to which someone tries to conceal it). We can eliminate the ones that are innocent or understandable, since we all do them, such as when someone wants to avoid confrontation (because of fear) or to ingratiate someone (to either get ahead or to be well-liked), as just a few examples.
So the exercise above says that we need to use a qualifier to eliminate what we don’t want in our definition, which is benevolent manipulation, so the descriptor malignant will be used, which is where harm is done to your well-being. So we are targeting the effects of manipulation in order to define its essential features. Note that there is no discussion on intentionality (which implies a degree of self-awareness) because that can be difficult to discern as some may be fully aware of their acts while for others it may be operating below their threshold of awareness.
Focusing on how the effects of manipulation can harm you (say by measuring how angry, betrayed or hurt the act makes you feel), we know we have to include at least two dimensions: underhandedness (deception for personal gain) and callousness (no regard to how it will affect you). Underhandedness is more descriptive than deceptiveness since it underscores that the act is usually done for personal gain. Let’s give it a formal definition.
Malignant manipulation is an underhanded (where it is hidden or misleading) and callous (where victim’s feelings are irrelevant) attempt to alter, even control, another person’s thoughts, feelings and behavior without their consent (which is coercive) for personal gain or advantage.
This leads us full circle to the defining features for those that score high for the traits in the dark triad: that is, they seek to maximize personal gain, at the cost of another and then to feel justified in doing so. In future posts, I’ll explore whether or not those of the dark triad have the need like we do to justify our actions.
In closing, it may be helpful to be reminded on what a healthy relationship consists of especially if we were ignorant of these guidelines in the first place, like myself.
Encourage expression of opinions. Say what they mean and mean what they say. Support you even if they don’t agree with you. Let you know in a direct and kind way if you’ve hurt them. Are capable of emotional intimacy—the mutual sharing of feelings and ideas. Trust others and exhibit behaviors that are genuine and authentic. 
 Birch, Adelyn. 30 Covert Emotional Manipulation Tactics: How Manipulators Take Control In Personal Relationships. Unknown.
 Davenport, Barrie. Signs of Emotional Abuse: How to Recognize the Patterns of Narcissism, Manipulation, and Control in Your Love Relationship. BOLD LIVING PRESS.
 Kole, Pamela. Mind Games: Emotionally Manipulative Tactics Partners Use to Control Relationships and Force the Upper Hand – Recognize and Beat Them (Emotional Freedom and Strength Book 1). Plaid Kilt Publishing.
 Sarkis, Stephanie. Gaslighting. Hachette Books.