My ex boyfriend can't keep me properly, not even letting me go! He says he'll self-harm him if i'd go away from him? What to do?

Broked up with him due to his toxic nature. Never again patched up but He isn't even Letting me go.
My ex boyfriend can't keep me properly, not even letting me go! He says he'll self-harm him if i'd go away from him? What to do?
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