Guild Wars 2

I partied with three other players last night to finish the HoT story. The boss fight went as was acting normal but we ended up wiping. In our second try the boss lost massive amounts of health in a very short time and we were able to defeat it before we converted our first Blightened companion. I don’t know what went wrong for this massive increase in health loss, but I thought it would be useful to post this experience here.

I’m on the 2nd vision boss and he is down to less then 1/2 health. And he is now immune to damage. What do I do? This definitely is a bug?