After the third pair of wizards have come out (Happens around 70% transmatting), for the love of all that is holy, step out of the zone.
Sincerely,
Everyone who knows how to turn the public event heroic
Edit: Killing the shriekers is what triggers the heroic event. If the transmat progresses enough, the shrieker basically opens and closes almost straightaway.
English
#destiny2
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If you can't take out the wizard and shrieker between 70-80% with people transmatting, you weren't very heroic =)