I am working on a book that features a form of "colonization" by destroying all life on a planet. The result of my first question related to this question : planetary-immolation resulted in a suggestion of Gamma radiation blast, that results from possibly aimed exhaust from a tuned fusion rocket. Now in regard to this idea what would the mechanism be and what would this look like from a local planetary POV (local inhabitant view)?
Known causes of high energy gamma rays capable of sterilizing a planet are from astronomical events, releasing far more energy than any mere K-1 civilization might be able to muster. The most likely candidates are the implosions of massive Type II Supernova, or the violent collision of neutron stars. Based on astronomical observations, these events generate "beams" of gamma rays with observed energies peaking at about 5.5 × 10^41 J.
It is perhaps possible this has actually happened far in Earth's past. There seems to be some evidence the Ordovician–Silurian extinction events 450 million years ago were caused by a gamma ray burst.
As to what it might look like, Wikipedia has a description here
Earth's atmosphere is very effective at absorbing high energy electromagnetic radiation such as x-rays and gamma rays, so these types of radiation would not reach any dangerous levels at the surface during the burst event itself. The immediate effect on life on Earth from a GRB within a few parsecs would only be a short increase in ultraviolet radiation at ground level, lasting from less than a second to tens of seconds. This ultraviolet radiation could potentially reach dangerous levels depending on the exact nature and distance of the burst, but it seems unlikely to be able to cause a global catastrophe for life on Earth
So the short term effect for those on the ground would be: not much. No dramatic events like the sky catching fire, or concrete buildings melting, in other words.
The true killer effects happen downrange of the burst impacting the Earth, when the high energy radiation changes the chemical composition of the Earth's atmosphere, stripping away the Ozone layer and filling the sky with photochemical smog and nitric acid rains down from the sky, poisoning the water supply.
The only means I can think of that would be within the range of a K2 civilization would be antimatter. Throw clouds of antimatter at the planet, when they hit the atmosphere you get very hot gamma radiation.
However, I'm having a hard time seeing this do more than frying the ozone layer. For the highest energy gamma rays I can find on a chart the atmosphere is basically a perfect shield. Unfortunately, the highest energy I can find an attenuation factor for is 500kev, some zeroes shy of the hot ones that will be emitted when the nucleus annihilates.