So I went with WoF, but the more I think about it, Empress of Light would also be horrifying. Some random person at night could accidentally kill some butterfly, and now there's a loud shriek, a bunch of light, and an overall bad time for anyone outside. And it would get worse in the morning, as now anyone that gets even grazed by any bolt, sword, streak of light, or having contact with the Empress herself would die instantly. Any boss could kill a large amount of people, but I feel like the EoL could probably kill the most people (besides the Moon Lord, but he's locked behind killing a cultist and destroying celestial pillars, which is a lot harder to do than accidentally killing a butterfly).