Battle of Dayos IV
The Battle of Dayos IV was an attack on the Klingon colony of Dayos IV by forces of the Zarnac Empire. The Empire launched its attack due to Dayos serving as the home for a settlement of exiled Zarnac - the Thirty-first House.
In mid-2378, a Klingon patrol ship on the edge of thier space suddenly exploded. At the time nobody knew the cause. It would not be learned until much later that the ship was attacked without provocation by the Zarnac Sustegaria class battleship IZS Resatalia.
The Zarnac struck suddenly and swiftly after arriving in orbit. Hundreds of Imperial Marines under the command of Codrac Drant, Eldest Son of the Twenty-forth House were dropped onto the planets only settlement. Once in position, the Marines set to their grisly work - slaughtering every Zarnac exile they found.
In what would come as a complete shock to the invaders, the Klingon settlers fought back, not only to protect their homes, but their reptilian neighbors as well. Drant - unnerved by this realization - asked that the attack be called off before his troops took prohibitively heavy casualties.
Canum Athon, Son of the Tenth House, the commander of the Zarnac forces, forcibly declined the request. This battle was his chance for true glory, and he would not have it snatched away by the "cowardice" of his subordinates. As such, he gave the order to massacre the Klingons as well.
The Attempted Genocide
Though he found the orders given to be obscene and disgusting, Drant carried them out with ruthless efficiency. The Imperial Marines laid waste to the colony, brutally killing anyone who stood in their way.
As it turned out, they would not have all the time they needed to complete the mass extermination. The Klingon Empire took quick note of the sudden communication blackout from Dayos. Not having any ships in close proximity to the colony, they asked the Federation to investigate. Starfleet dispatched the closest ship - the USS Hyperion-D - to the area.
When the Resatalia detected the Hyperion on long range sensors, Athon found himself having to make a tough decision - pull out his troops and leave without completeing their objective, or risk engaging Starfleet in open combat? As he had been given explicit orders not to attempt a confrontation with Starfleet, Athon gave the order to recall the Marines. Once all were aboard, the Resatalia engaged its barrier field and fled the system.
The Vultures Swoop In
After the Zarnac left the area, the decimated colony was ripe for plunder by pirates operating along the Federation/Klingon border. One such group chose to take the chance, landing several men to loot the remains of the Dayos colony.
They did not make a swift enough exit, however, as the Hyperion arrived on the scene shortly there after. As her fighters chased off the pirates ships, an away team was sent down to assess the situation.
What they found was chaos. The spaceport had been destroyed, and most of the buildings in the settlement were leveled. They were forced to engage the pirates left behind on the planet, thinking them responsible for the carnage. It was soon learned however that the pirates had not been involved in the massacre.
It proved lucky that the Hyperion arrived when it did. Nearly half of the Zarnac settlers had been killed, as well as a quarter of the Klingons. Among the dead were Dahar master Karn and his son Krag (mentor and childhood friend, respectively, of then Lieutenant JG Drakt, the Hyperion's deputy security chief), both of whom gave their lives valiently protecting the Zarnac settlers.
With the entirety of their leadership destroyed, the Thirty-first House looked to Drakt for leadership. They nearly demanded that he resign his Starfleet commission and assume leadership of the House. But Drakt, whose parents were among those killed, declined - his only remaining "family" was aboard the Hyperion.
It was expected that, when word reached Qo'noS, the Klingon Empire would immediately retaliate. However, the Empire was still weakened after the Dominion War. As such, no counterstrike was ever attempted. The Klingons did, however, provide logistical support to Starfleet Intelligence the following year when it boldly placed Drakt on Zarn as a deep cover operative.