The Rain That Shook The World is Esper Zero's signature arc in Neverness to Everness, and as of writing, players can only get it by purchasing a premium pack from the in-game store. The pack in question is called the Limited Press Arc Pack, and you'll also get a bit of extra gacha currency along with The Rain That Shook The World. Thankfully, the pack itself is pretty cheap, and if you're not sure whether you want to commit to NTEyet, the Limited Press Arc Pack might still be worth getting.