Yuusha ga Ochiru Toki: Kyouran no Mitsu

The protagonist called Thor Beleth is an ace adventurer in the guild.
One day, when he comes home after a quest, he finds that his sister
named Eclair absconded from their house leaving a letter to him.
The letter tells him she is being involved in something bad and
immediately drives him to go looking for her.
Will he be able to take Eclair back?

[Modified from DLsite English]