Fushigi no Kuni no Succubus

Based on the erotic doujin game by Muni Koubou (むに工房).

A boy falls down a well and ends up in a whole new world -- and before him sprawled a huge castle full of busty succubi! By borrowing the power of Louise, a girl that suddenly appeared in his hour of need, he hopes to find a way back home... however...

(Source: DLsite English)