The story is told from the perspective of the leader or military commander of an aggressive alien race, currently at war with humanity. He is aboard the flagship of the alien armada, which is currently in the Sol system.
The alien race is described as cold-blooded, with taloned hands and lamprey-like mouths. There is a passage where a thrall or other servant brings a platter of (for lack of a better term) alien Hors d'oeuvres. The commander skewers a tiny rodent-like creature, which I believe is described as warmblooded, with one of his claws. He then feeds on it, draining the blood out of the creature. I believe there is mention of vapors/gasses in the ship's atmosphere that would be toxic to human beings.
The commander is frustrated with the progress of the war. His race holds an enormous technological advantage over humans. His side has FTL drives, and a devastating mass-to-energy converter weapon. As luck would have it, though, their feared weapon is useless against Terran vessels. The simple radiation screens human rockets use to protect their fragile crew from the rigors of space has had the unintended side-effect of neutralizing the effects of the mass-energy conversion beam. As such, the aliens have been forced to resort to old, outdated technology like nuclear missiles and what not. This means the Terrans and the alien race have effectively equal weaponry. As such, the war has been something of a stalemate to this point.
The story takes a sharp left turn with the arrival of a third race of creatures. A pair of aliens, possibly child-like or juvenile for their species, appear near the sun in a tiny space vessel (or possibly, two small vessels). Somehow, the alien is able to intercept or eavesdrop on this new arrival. It is with great alarm the alien commander discovers these two are somewhat bored with things, and are thinking about causing the sun to go supernova to see what will happen. I don't recall too much about these new aliens. I got the sense (back when I first read the story) that they were intended to be a semi-comical take on the apocalyptic figures of Gog and Magog. I'm not sure about that, though.
I get a little fuzzy on how the alien commander tries to deal with these creatures. At some point, he becomes desperate, and orders one of his battleships to engage FTL drives and pop out of hyperspace right underneath the tiny ship. The commander then orders his fleet to fire on his own battleship with the mass-energy converter weapons. The battleship masses many thousands of metric tons, and the sudden conversion of it to pure energy results in a blast of staggering proportions. The alien vessel is completely unharmed.
I don't recall what prompts the third alien race to leave without blowing up the sun. It may have been boredom, or they simply decided that the people here were rude and noisy (what with the exploding battleships and what not). I do recall that the alien commander immediately orders his officers to contact Earth to discuss peace negotiations. The reasoning being, with beings as powerful as the ones that nearly destroyed the sun in the universe, their conflict with the humans would have to be put aside.
Timeframe of Publication
I think this one came out in the mid-80s. That's when I recall reading it. So, 80s or earlier. I'm almost certain I read it in a sci-fi magazine, and not in a paperback anthology or the like.