As infuriating as this situation may be, once someone very high up takes action, you don't really have any choice but to move forward. You can't buck the system now.

Does the new employee report to your manager? If yes, I would let your manager handle the situation.

If no, you and your manager need to come up with a strategy to manage this other person -- this isn't impacting just you, it's also impacting your manager.

OP you submitted to the new employee when you passive-aggressively suggested going to your boss to get their opinion, rather than simply asserting yourself. Don't be infuriated, you gave away your power it wasn't taken from you

sounds like my old coworker. You can try to have a conversation (one on one) but toxic people don't change. If your convo doesn't work, steer clear!

A relatively new employee at work has been given a lot of responsibility and is developing a reputation for creating a toxic dynamic. Recently this person had a conversation with me that they did not agree with how I was handling a situation with a client, which I discussed with my boss prior. I suggested maybe we should go to my boss and get her take. Instead they went to someone high up n the company, discussed the situation, and my this person called a private meeting with me about it and didn't include my boss, which is generally frowned upon as it left me without a defender in the room. It infuriated me and my boss wasn't too happy about it either. I got told I had to handle ti difficult. Is it best to confront someone like this or just steer clear of them? How do you confront them in a way that doesn't seem, well, confrontational and paranoid?