kankaku shadan
Kankaku shaden (lit. "Sensory deprivation" or "Sense blocking pit") is a fetish consisting of a character falling into a tentacle pit and being ravaged by tentacles, while being unaware of what's happening to them due to having their senses cut off, usually by either magic or paralyzing poison.
Often includes a follow up where the sensory nulling ends for one reason or another, which causes the victim to be forced to feel intense senses of overwhelming pleasure all at once. Breast expansion and inflation are often present as well.
The fetish originates from Danbooru post #6364507 by Shirabe Shiki, which also gives the fetish its name.
In rare cases when a tentacle pit is not involved but the setting is otherwise similar, the tag is still applicable.