Xhul'horac Phase Transition

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I just had a question about phase transition on this boss. It's said in most guides that when you kill the big add (Vanguard), you transition the boss into phase 2. I was wondering if you just kept the add alive (using 3 tanks) could you remain in phase 1 forever? Or does the boss phase switch on health percentage, not on add death?

And I know the room would be filled with a lot of fire, I'm just curious about the mechanic here

Thanks!

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the boss and the add share health, so hitting the boss is eventually going to kill the add too

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the boss and the add share health, so hitting the boss is eventually going to kill the add too

Ok did not know this, thank you
Can close this post or w/e