I don't recall it explicitly mentioned that Asher's arm turned into a Vex appendage due to exposure to the fluid.
I thought it was more along the lines of they experimented on him and cut off his old arm and replaced it with one of theirs. And that perhaps it was starting to grow.
Now granted, doing so probably exposed him to the fluid.
I just doing think it was a simple matter of the fluid turning his old arm into his metal one.
BUT I'm on my phone. And looking this stuff up on a phone is a pain in the arse. So maybe I'm remembering wrong.
English
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well when u kill the hobgoblin on steroids strike he rants and asks something along the lines of "did you cut off its arm and send it spiraling into the abyss" soo thats probably what happened
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I think what you said near the bottom is right but we do know that his arm was totally transformed not merely replaced since if it was then when his Ghost respawend him he'd be back to his normal state but he still comes back with his vex arm so it is entirely written into his DNA.