The unpredictable, unstable, obsessive EX

It is always hard when a relationship ends, a divorce or break up takes place. But when the relationship was/is toxic, it is in the best interest of our Emotional Health and Stability to end toxic relationships. 
Toxic individuals do not change. They may seek counseling, but they do not seek it because they genuinely want to change and be better persons.. The only reason a toxic person agrees to counseling or help, is so that their victim "believes" that they are trying hard to change. Once they have their victim believing that they have changed, the abuse (lying, manipulating, cheating, stealing) will start again, and continue until they are caught again. 

TOXIC PEOPLE WASTE SO SO SO MUCH OF OUR TIME, AND THEY SIMPLY DO NOT CARE. You cannot make them feel bad or remorseful, because they do not have a conscience. They lack inner depth and love. When a person does not have consciousness and awareness of the feelings and pain of others, they are capable of terrible and evil things. 
Just look at how they recklessly live. They have no concern for their own life. Much less the life of anyone else. 

When the toxic relationship ends, because you have had enough. Even though you are able to let go and you are ready to move forward in life, doesn't mean the toxic person is.  
Even if it is the toxic person ENDS the relationship, and moves on to another person. They will still feel the need to punish you every chance they can get.  Especially if you decide to move on as well and begin to see other people and date again. Once the toxic person finds out you are moving on and thinks you may even be Happy, that is when they may become obsessed with getting you back, reconciling, or just making your life miserable. 

This behavior becomes an obsession.  Your ex develops an unhealthy and dangerous obsession with you. If you have children with this person, your situation can become so miserable that you begin to feel hopeless. 

This is called the obsessive EX syndrome. It is when your Ex-Wife/Husband will stop at nothing to make your life unbearable and miserable. They will even use their own children as mass weapons of destruction to punish you and make your suffering even worse. 

Even if the ex has moved on, even re-married. Often times, the obsessive ex that you are dealing with wants you to still give to them, do things for them, and be around when they call.  They are very delusional, they truly believe that they are allowed  to move on with their new partner, but they do not believe that YOU (their Ex) is allowed to move on or be happy. Once your obsessive ex finds out that you have found someone else, they become obsessed with making your  life hell. They do not stop with just tormenting you.  They also will harass and stalk the man/woman that you have moved on with. They will use the children that you share to hurt you. They do not CARE, the psychological damage they are causing their Own children. As long as they are in control. They want to control You. This is usually because the  obsessive Ex realizes that if You find someone else, they will lose the control they have had on you for so long. 
This type of behavior is delusional, and it can become very dangerous. They may not harm you physically but they find Many other ways to harm you. Such as accusing you of abuse, physical and emotional, they will turn the children against you, and slander you to friends, colleagues and the entire community. An obsessive ex knows what they are doing, but justifies it to their own self. They are selfish, but are too self centered and narcissistic to take responsibility for anything in their life that they are doing or that they have done.  These people do not always have a chemical brain imbalance. They usually just lack compassionate for anyone, anything, except themselves. They have a sense of entitlement, and they expect impossible and unrealistic 
expectations for everyone — EXCEPT FOR THEMSELVES! 

This can be extremely hard for you, but you must be strong. I have found that once the children get older that you must apply the NO CONTACT RULE. This is helpful. But it takes a significant amount of time before your unstable ex leaves you alone.