Based on the erotic game by Lune.

Alfheim is the only female Elf world where they live in the place of the Sacred Tree. The war between the Elf and the Dark Elf has caused making Elf casualties increasing, plus the strength of the Sacred Tree getting weakened so that it can not provide any more children. The only way is to find men who can give a descendant to restore the Elf race population. Yuuichi accidentally met with Loli Chairman of the Elf Village tribe named Nol. Then, Nol invites Yuuichi to Alfheim to help her make a descendant of the Elf race.

(Source: Translated and Edited from Nekopoi)