At 400m diameter we should expect an impact energy of ~3Gt, which would be one of the most energetic events in human history, if not the most. For a comparison

At 400m diameter we should expect an impact energy of ~3Gt, which would be one of the most energetic events in human history, if not the most. For a comparison, the Lake Toba eruption (which is suspected of causing homo sapiens to almost go extinct 76kya) is estimated to have been in the 1-2Gt range.