This is a short story, I think by Ray Bradbury or Sheckley.
Two (?) adventurers/archaelogists are inside of the weapon cache left by Martian civilization which was destroyed by war. Eventually they find a chamber marked "Absolute weapon" (or ultimate weapon). They fight over who will get it, and only one of them survives. He opens the lock, releasing sentient blob that eats his fallen companions and then him, as well.
"The last weapon" (1953) by Robert Sheckley, available at the Internet Archive.
Two (?) adventurers/archaelogists are inside of the weapon cache left by Martian civilization.
Edsel was in a murderous mood. He, Parke, and Faxon had spent three weeks in this part of the deadlands, breaking into every mound they came across, not finding anything, and moving on to the next. The swift Martian summer was passing and each day became a little colder. Each day Edsel’s nerves, uncertain at the best of times, had frayed a little more. Little Faxon was cheerful, dreaming of all the money they would make when they found the weapons, and Parke plodded silently along, apparently made of iron, not saying a word unless he was spoken to.
Eventually they find a chamber marked "Absolute weapon" (or ultimate weapon).
At the end of the room was a door. There were letters in Martian script engraved on it.
“What’s that stuff say?” Edsel asked.
“Something about ‘final weapons,’” Parke told him, squinting at the delicate tracery. “A warning to stay out.” He opened the door. Both men started to step inside, then recoiled suddenly.
Note that a second mention of said weapons is made later on, this time with the titular "last weapon":
“With those troops — the force-field — and the weapons — I can take over the world.” He said it coldly, knowing it was a fact. He turned to go back to the cave, to activate the Synthetics, but stopped first to pick up the little black box Edsel had carried.
Engraved on it, in flowing Martian script, was, “The Last Weapon.”
I wonder what it could be, Parke asked himself. He had let Edsel live long enough to try out all the others; no use chancing a misfire himself. It was too bad he hadn’t lived long enough to try this one, too.
He opens the lock, releasing sentient blob that eats his fallen companions and then him, as well.
Well, he told himself, let’s see what the Martians considered their last weapon. He opened the box.
A vapor drifted out, and Parke threw the box from him, thinking about poison gas.
The vapour mounted, drifted haphazardly for a while, then glimmered white in the dying light, and Parke saw that it was just a tremendous mouth, topped by a pair of unblinking eyes.
“Ho ho,” the mouth said. “Protoplasm!” It drifted to the body of Edsel. Parke lifted the blaster and took careful aim.
“Quiet protoplasm,” the thing said, nuzzling Edsel’s body. “I like quiet protoplasm.” It took down the body in a single gulp.
A protoplasm, if I am to believe Wikipedia, is "the living content of a cell that is surrounded by a plasma membrane".
I found it by CTRL + Fing this page for weapon. It's a page containing a short description of all Sheckley's stories, which I had found when answering Short story by Sheckley about a man from the future who radiates love and lust.
