If this is death by eldritch horror, then it should be pretty bad. Just a simple decapitation isn’t going to work.
You mentioned tentacles so let’s go with the obvious.
You run, but the writhing tentacled mass continues to keep up the pace despite its girth taking up the entire hallway which is seemingly endless at this point. You keep an eye out for a window or door, anything that could get you out of here, but there are no other exits except the one ahead of you.
You start to hear voices mocking you about past wrongs you have done or even wrongs you could have prevented. You feel a crushing sense of guilt, even from the ones you didn’t normally feel guilty about. This guilt quickly turns into despair. You cry out at these inhuman chattering voices to shut the hell up, but they just laugh and continue to mock you.
Meanwhile you start to feel the mass behind you getting very close. The exit is right there before you, but you’re so overcome by the sheer hopelessness of it all that you just stop and don’t open the door. What is the point? You can’t escape. You deserve to die and worse. You truly believe that now.
A single tentacle touches your arm and then another. Another then wraps around you leg and then around your waist. You feel it squeeze tightly around you and pull you in.
Your body is slowly absorbed by the tentacled mass, but it isn’t melting you or anything like that. It’s making you part of it. You’re being completely torn apart but left alive to feel the pain of all of it. The sounds of mad laughter flood what is left of your mind.
Everything about you is gone. You’re multiple bloody pieces of mush in constant flux of shifting and changing within this thing’s body which has long since forgotten about you, indeed if it even considered you as something to be thought of at all. You were an insignificant meal at best.
Years, decades, centuries pass, but you’d never know it because time is meaningless in your condition.
Every once in awhile though when the thing is in a state of dormancy, a sliver, a minuscule piece of you somehow regains something like a glimpse of a memory and an eye manages to make it to the surface of the thing’s outer skin.
A single tear weeps from it.
Followed by a thousand laughing screams.