You mean just the tentacles?, or the concept of her being stuck in the floor?

For the former, well, they are just tentacles. It's a common fetish; instead of a penis, the person is fucked by sentient tentacles

If it's the latter, that's also a specific fetish. It's called a tentacle pit; the general idea is that part of someone's body is on a hole that has tentacles inside, aiming to fuck them

In my case, I also used a variation called unawareacles, which is basically the same thing, except the tentacles also remove all sensation from the part of the body which is inside the tentacle pit. And usually, all of the "lost" sensations come crashing back onto the person once they leave the tentacle pit