Career advancement
I'm still a little confused. All in all, you accepted this new job offer due to its opportunity, reputation and career which you believe you're able to build in future. Meaning you're accepting a pay cut because you're investing in your career in advance? Is that the case?

Work bully
Perhaps back in the old workplace, you were reporting to this senior and he had the chance to bully you by giving more work, blaming you for certain things etc. What about the need to work with him laterally now? He's not your superior, and you've made this fact known to your management and HR, perhaps you can deal with him?

Consider this a good opportunity to deal with the traumatic experience. You can confront him about what he did in the ex-company and demand an explanation or apology, or, work amicably with him showing that you have no fear towards him and the fact HE should be afraid 'coz you know his true color and you've got dirt on him.

If your instinct is screaming Fight or Flight, and you're choosing flight - reconsider whether its worth sacrificing your work and career because of this bully. I suggest to seek a 3rd party counseling or psychology advice to deal with it and see how you can neutralize the situation. Leaving your current job without another offer on-hand is not wise, despite having no real commitment.

In a funny twist, I've dealt with work bully before. I've suffered, I've broken down and then rebuilt myself and find ways to deal with the bully. No doubt, I was also "lucky" in a way there was higher intervention who noticed the situation and some sort of remedial action was taken. Therefore I was able to manage the person in question, and FYI, still working under him (no doubt, he has changed quite a fair bit since then as well). The scar will always be there, and it'll never go away but putting personal feelings aside, work is work.