Kuro Elf ni Kawareta Ore no Dungeon Seikatsu: Sanshoku Furo to Jigoku-tsuki

One day, a "calamity" comes and destroys the city, killing both the protagonist Kruz and his sister Arti. However, Before Kruz dies, he sees a dark elf girl named Enju whom he had met before. The girl says. What do you want, to end your life silently like this.... or to become a shinigami who kills "calamities" ?