Zalmarath boss fight glitch

While I was attempting Zalmarath,d he suddenly just stopped taking damage, prior to this it was fine so I'm not entirely sure what happened.

Logging out and back in doesn't change anything, nether does going back to the caravan or anything, I'm not sure what to do about it but it is a bit of an important fight as you need it to progress, I will update if I find anything new out.