The One Who Waits is about souls that live in a well on Mars. The story takes place far in the future, 10,000 years after there's no more life on Mars. A few guys from Earth land on Mars and go to the well to try the water. The creature that describes himself as "mist" comes out of the well and possesses one of them. The guy fought back for control and ended up dying. The creature then possessed another guy and shot himself. He then possessed the rest of them and trapped their souls in the well. The soul waits for more visitors to repeat this.
Bradbury's Stories by, Scarlett Fortney, Summary of The One Who Waits
The story involved a creature or alien telling the story from a first person perspective, how he sits and waits and is all alone, until either astronauts or space marines show up.
I live in a well. I live like smoke in the well. Like vapour in a stone throat. I don't move. I don't do anything but wait. Overhead I see the cold stars of night and morning, and I see the sun. And sometimes I sing old songs of this world when it was young. How can I tell you what I am when I don't know? I cannot. I am simply waiting. I am mist and moonlight and memory. I am sad and I am old. Sometimes I fall like rain into the well. Spider webs are startled into forming where my rain falls fast, on the water surface. I wait in cool silence and there will be a day when I no longer wait.
The One Who Waits
The story then shifts to a third person narrative of the astronauts/marines as one by one they're possessed by this thing, causing the eventual in-fighting and murder of each crew member.
The sound of water in the hot sunlight. Now I hover like a dust, a cinnamon, upon the soft wind.
"What's the matter, Jones?"
"I don't know. Got a terrible headache. All of a sudden."
"Did you drink the water yet?"
"No, I haven't. It's not that. I was just bending over the well and all of a sudden my head split. I feel better now."
Now I know who I am.
My name is Stephen Leonard Jones and I am twenty-five years old and I have just come in a rocket from a planet called Earth and I am standing with my good friends Regent and Shaw by an old well on the planet Mars.
I look down at my golden fingers, tan and strong. I look at my long legs and at my silver uniform and at my friends.
"What's wrong, Jones?" they say.
"Nothing," I say, looking at them. "Nothing at all."
ibid
In the end, the creature ends up back in his pit, waiting.
I live in a well. I live like smoke in a well. Like vapour in a stone throat. Overhead I see the cold stars of night and morning, and I see the sun. And sometimes I sing old songs of this world when it was young. How can I tell you what I am when even I don't know? I cannot. I am simply waiting.
ibid
It was earlier answered here.
