Oops, you killed the end dragon. Turns out, she was actually protecting all the realms from a terrible threat. After she is killed, Thaumcraft chaos or Abyssalcraft Bad Stuff starts coming out of end portals. Maybe Botania mana becomes corrupted. Maybe a miasma spreads over the earth, blocking the sunlight, and hostile mobs start to spawn during the day - not just one or two, but skeleton armies. Maybe around the end portals, cursed earth stats spawning in an ever-expanding radius.
The cure is to fire Astral Sorcery starlight into the end portals. Of course, your first have to expose those portals to the sky. Building the multiblock (with maxed-out celestial crystals attuned to each of the constellations) might be a bit of a challenge, too.
Hmm, this could be a nice mod, but would need more details. Also, would need a vanilla disaster as well as a way to cure it, just so it doesn't force you to use mods (which would become required to even use the mod). Like perhaps the dragon was holding back the tide of evil endermen, and only a few trickled into the Overworld and Nether. Then killing the dragon was like breaking the dam, and a tidal wave of endermen flowed through the portal into the Overworld, with a hunger for foreign blocks. This made it much worse, with them being aggressive on sight, wrecking the landscape and killing off potential threats, you being one of them. The only way to stop it was to arm the villagers and destroy all 3 end portals to stem the flow. Any remaining endermen would have to be slain.
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