Everything that you have killed over the years in video games have now joined a super-alliance to destroy the Earth. This includes bosses as well.
This includes everything from the tiny 8-bit sprites to the colossal Reaper Destroyers.
How screwed is Earth?
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Well I'm not looking forward to seeing all those Heide Knights I farmed for their armour in Dark Souls II, since I used like 25 bonfire ascetics and they are more powerful than most of the bosses now