The event is glitched about 50% of the time even if you kill both wizards the shreiker will open then close really fast making heroic impossible. This should not surprise anyone as all bungie does is screw things up.
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That's because the timer got beyond 80. No matter what you do, if you kill the wizards and people kept the timer running and it gets past that, the shrieked opens and immediately closes. That's part of the heroic challenge: knowing when to stop progress.
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I see well that's good to know i guess. A rather dumb restriction though 75 - 80 is not very long.
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Watch your timer more closely, wizards spawn at 71, not 75. A smart group knows to get out of the circle as soon as those spawn, then the timer no longer progresses.