Kashoku no Ten

At the urging of his friend, the main character, Itsuki Kashima, is sent to clean up his friend's relative's house deep in the mountains. On the way, he gets lost and wanders into an old shrine, where he encounters something hideous inside the main shrine and is attacked.
Trapped by the tentacles, Itsuki gradually drowns in the pleasures of the tentacles.

[From Steam]