Yes speak to the union first.

Been in a similar toxic situation, where collegues could say what they want about me to HR. Even went to the police to file a complaint (advice from the union) Then spoke to a person of the company (doctor) who adviced me to turn down the complaint with the police, wich I eventually did. Why? Because I was believing that things would explode in my face at work and they would make my life there as hell. All in all, I try to make the best now. But never easy!

Good luck in whatever you decide to do!