触手の館～快楽に捕らわれる女子校生～（Shokushu no Yakata ~Kairaku ni Torawareru Joshikousei~）

A revenge tale about a bullied student who fuses with something otherworldly...

Squirming tendrils are the living bondage that he can summon at will
to tentacle r*pe the girls who kept him down.

I was always teased at school, and no one helped me. I cleared my head by taking walks in the desolate "lost woods", where one day I came across a mansion. Inside were tentacles... and they seemed to like me.

Like a deer in headlights I froze while the tentacles lunged.
I expected to die right there.

But the tentacles stopped.
"What? You're not going to eat me?" Of course the tentacles said nothing.
"Well," I ventured, "maybe you can listen to my story."

Later, it happened that some of the girls who teased me stepped into that mansion.
The tentacles attacked them. "I guess you sympathize with me...
Well then, maybe I could borrow your power?"
That's how I made an unlikely friendship... and began my revenge.

Tentacles ravish heroines who can't escape!
The protagonist wields bondage-like tentacles in scenes of face rape,
defloration, anal, double penetration, and more.

[From DLsite English]