Game no Sekai ni Bakemono to Shite Tensei Shita Ken ni Tsukimashite.

The only thing that washes away the daily stress of your factory job is that of MMORPGs.
Taking control of that carefully-sculpted and ideal girl fills you with life.
While playing the game...you fall asleep, and suddenly awaken in the starter dungeon!
And you're a tentacle monster!
Then, that ideal girl you spent hours customizing appears before you!
It's time to put your newfound tentacle powers to the test!
You're a tentacle monster, after all; it'd be rude NOT to violate her.

[From DLsite English]