Kdjykuj

Currently, I have two examples in my mind:

• Halo (Halo video game series; 2001): Can destroy all sentient life within three galactic radii




• The Moment/ Eye of Discord/ Galaxy Eater (Doctor Who (2005) TV series; 2013): Can destroy entire galaxy within a moment

Which Sci-Fi work first showed weapon of galactic-scale mass destruction?

1930: "The Triple Ray", a short story by R. V. Happel in Amazing Stories Quarterly, Fall 1930, available at the Internet Archive.

Plot summary by Everett F. Bleiler in Science-Fiction: The Gernsback Years:

Professor Lucius Raymond is the inventor [of] two rays, the Twin Ray and the Triple Ray. The famed Twin Ray, a combination of ultraviolet and infrared, amounts to a disintegrator. Its source is an adapted Crookes' tube, with an unmentioned radioactive substance. A demonstration of this ray to the Germans leads to their retreat from Paris. (While the author is not clear, this seems to refer to a second world war.) This ray, in addition to being the prize weapon in the American arsenal, has extensive peaceful uses.

The Triple Ray, which destroys atoms, has been discovered almost by accident. On the single occasion that it is used, it drills a hole through a hill. More, it continues on and slices the top off a nearby mountain. Then it leaves the solar system and disrupts several stars. And it continues, moving faster than light. Since the universe is finite and space is curved, the ray will continue on around the universe, spreading, and return to disintegrate Earth. It will eventually devour the whole universe.

Excerpt from the story:

My curiosity vastly aroused by his letter, I took the first opportunity to see my old friend and learn what discovery had so upset him. Of the truth of his contentions I will say nothing other than that in so far as I was able to follow him in his deductions and experiments every fact seems to bear out his theory. His first discovery at the observatory was that the tremendous energy released from the mountain lodge, while he was attempting to measure the speed of the Triple Ray, had formed into an interstellar ocean of destruction, rushing madly through outer space, engulfing all matter it encountered and converting it into its own destructive nature.

It was, indeed, in the very same lodge only a few days before his death that Lucius, flat on his back from two strokes which left him paralyzed below the waist, slowly and carefully explained to me fully for the first time the exact nature of his discoveries at Dudley. I will give this in his own words, since it is thus that I best remember what he said to me.

[. . . .]

"It is for this reason that I cannot say how far in space the thing has traveled nor where it may be now. But this I do know and have proven many times over. The ray is traveling, not in a straight line after all, but instead in a closed circle, and must by every law of mathematics return again to its beginning. And since I have been able to learn by experiment that it renews and increases itself by that which it destroys, I have no particle of doubt in my mind but that on its return to the earth our planet will be utterly annihilated.

"The circle is vast, I know, for it was months after I first began to trace its course on an astronomical map that I was able to detect the slight deviation of the arc. It must travel the very fringes of the known stellar spaces where it takes light a thousand million years but to cross. Yet so terrific is the ray's speed that it may carry it round and back, I fear, in a lifetime; perhaps less. And no sight of its return will give warning, since it precedes its own light as lightning seems to precede thunder. It is, in short, a natural force which will surely ride the universe until all active matter has come within its circle, as it must sometime, and been destroyed. And even then it will circle on until all time has ceased to be. Perhaps it will finally be the birth itself of a new and different universe. I do not know.

Sounds like space opera to me! E. E. "Doc" Smith's Skylark DuQuesne, 1965:

Dorothy’s face began to pale. “By that analogy you mean destroy the
whole galaxy! How can such a thing be possible? It can’t possibly be
possible!”

He told her how the operation could be performed. That apparatus that
the Barlo women had dredged up out of nowhere had a lot of capabilities
that did not appear on the surface. Blackie DuQuesne had perceived one
set of those possibilities, and he and Blackie had been working on the
hardware. They were calling it Project Rho.

The effects of Project Rho were to destroy two galaxies. One galaxy was destroyed by removing most of its stars:

In Galaxy A, where billions of suns had gone through
the stellar cycle of evolution for billions of years, there was scarcely
a corporal’s guard of primaries left. It was a strange, almost a
frightening sight. For with the loss of the suns the composition of the
galaxy had changed to something never before seen in all the plenum of
universes. Nearly every sun had had planets; nearly every planet
remained behind when its sun was stolen. Now they roamed at
random—uncontrolled, barren, uninhabited—lacking not only the light
and heat of their primaries, but freed from their gravitational reins as
well.

The stars from galaxy A were teleported to the galaxy of the Chlorans, which in turn caused all its stars to explode:

Thus millions
upon millions of Chloran planets were destroyed without any intelligent
entity either giving or receiving warning that an attack was being made....

They died in
uncounted trillions. The greeny-yellow soup that served them for air
boiled away. Their halogenous flesh was charred, baked and desiccated in
the split-second of the passing of the wave front from each exploding
double star, moments before their planets themselves began to seethe and
boil. Many died unaware. Most died fighting. Some died in terrible,
frantic efforts to escape...

But they all died.

In the Star Trek episode "The Alternative Factor", 30 March 1967, A highly unscientific threat to destroy our entire universe plus an alternate one was revealed, destruction on a scale that makes mere galaxy-wide destruction seem infinitesimal:

KIRK: What's going on? This leaping from universe to universe. This wild talk about a murdering creature who destroys civilisations What's the purpose?

SPOCK: Jim, madness has no purpose or reason, but it may have a goal. He must be stopped, held. Destroyed if necessary.

KIRK: I don't follow you.

SPOCK: Two parallel universes project this. One positive, the other negative. Or, more specifically, one matter, the other antimatter.

KIRK: Do you know what you're saying? Matter and antimatter have a tendency to cancel each other out. violently.

SPOCK: Precisely. Under certain conditions, when two identical particles of matter and antimatter meet.

KIRK: Like Lazarus. Identical. Like both Lazarus', only one is matter and the other antimatter. If they meet.

SPOCK: Annihilation, Jim. Total, complete, absolute annihilation.

KIRK: Of everything that exists, everywhere.

And when Kirk meets the sane Lazarus:

KIRK: Antimatter?

LAZARUS: Here, yes.

KIRK: And if identical particles meet

LAZARUS: The end of everything. Civilisation, existence, all gone. I tried to stop him, Captain. That's why I took your dilithium crystals.

Of course "The Alternative Factor" was aired in 1967, after Skylark DuQuesne (1965) and The Star Kings (1947) were published.

Possibility might be a weapon in Edmond Hamilton's novel "The Star Kings." This weapon was the "Disruptor," able to annhiliate vast regions of space itself and hence all matter within it.

A somewhat different kind of galactic-scale destruction occurs in "M33 in Andromeda"* (1943) by A. E. van Vogt, which later became the climactic encounter of the fix-up The Voyage of the Space Beagle.

The relevant part of the Wikipedia plot summary:

In the last section, Anabis, a galaxy-spanning consciousness, is encountered. Once again, it is both malevolent, starving and aggressive, and under all circumstances must be prevented from following the ship back to any other galaxy. Anabis, which is essentially a galaxy-size will-o'-the-wisp, feeds off the death of living organisms, and has destroyed all intelligent life in its galaxy. It transforms all planets it can find into jungle planets through terraforming, since it is these kind of worlds that produce most life.

*The Andromeda galaxy is actually M31. M33, the Triangulum Galaxy, is the third-largest galaxy in the local group, after Andromeda and Milky Way.