Powder blue in my opinion are the most aggressive tang in this hobby. People crown the Sohal constantly but I've never seen a Sohal come close to the aggression PBT display. Some PBT are not aggressive, IME 1/4 are pretty docile, 2/4 are nasty as all heck, and 1 is aggressive but only to newcomers or intermittently.
Powder blue and Achilles in my experience (Achilles odds are usually 1/3 or 1/4 are nasty in my experience) hold permanent grudges. Most tangs will fight new tangs and give them heck for a day or a week. Time and time again I expected PBT and even a couple nasty Achilles I have had over the years to "forgive and forget". It doesn't happen. Once they decide they have a beef, they'll take it to the grave. I've waited months for one to learn to tolerate new additions and it just doesn't happen and ends badly.
My purple tangs, sailfins, naso, hippo, chevrons, Kole, clown, Sohal, Caribbean blue, and several others who were tank bosses or harassed newcomers would eventually get over it and accept new additions - even of the same genus or species! This was NOT in the cards for PBT or the rare nasty Achilles, (who are every bit as bad as the nasty PBT).
You have to replace and try again for any hope.
I added my evil Achilles to a large angel tank with larger dominant tank bosses and more aggressive fish. He left the resident yellow belly hippo tang alone and the emperor angel and passer angel boss fish didn't take any flack from him and didn't allow him to ever assert dominance. I was honestly pleasantly surprised that someone could put this guy in his place.
I've got two moderately to significantly aggressive PBT currently but they were added at the same time as all of my other tangs. I did add a goldrim tang after the fact and they harassed him minimally but with 10 tangs in that qt there's not really opportunity to harass any one fish that much. I've sold evil PBT in the past.