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As they come down from Olympus, Zeus sends Iris to warn them that if they do not turn back, Zeus will harm Athena horribly. Iris adds,
He (Zeus) has less words of wrath or indignation to waste on Hera: who habitually defies his decrees. But you’d be dreadful in your brazen impudence, if you truly dared to raise your great spear against Zeus.’
I think Iris adds this to make the warning personal for Hera.