(Disclaimer: while writing this I have little to no idea about any of this and most of this is a massive middle finger to accepted cannon. You have been warned.)
Onóir was, prior to its exterminatus at the hands of a two-stage cyclonic torpedo, an incredibly resource rich planet in the Segmentum Pacificus, constantly under heavy rainstorms, rich in nutrients, causing Onóir to have an extremely fertile landmass. It was the birthplace of the Onóir First and Foremost, who now reside in an imperial warship, An Réamhtheachtaí, "The Forerunner". They have an intrinsically militarised population and are led by Lord High Admiral Tiberius Scelt.
Prior to the discovery of Onóir by the Imperium it was a mostly Terran like planet with a single landmass encompassing its equator and two frozen icy wastelands at its poles, and was home to a variety of extremely vicious predators who decimated the original colonial party and ensured that the poles remained unclaimed to the day of its destruction. The equatorial landmass, while still being deadly, was more hospitable to the colonies and contained more herbivores than carnivores and held many unique kinds of flora, many of which survived the destruction of the planet and can be found on the Forerunner.
The original colonials became extraordinarily rich off mining the planet's many ores and harvesting the local plants which, thanks to a unique mineral found in the rain, could be liquidised into a potent fuel. Unlike the vast majority of the Imperium, Onóir wasn't a hellhole to live in and actually had a competent ruling class and whose PDF leaders actually used tactics that prioritized troop survival over enemy casualties. This was undermined by the fact that ruling class were Nurgle worshippers and the PDF leaders refused to acknowledge that the Hive Fleet and Necron Fleet were an actual threat.
As a result of this, the planet was royally screwed over, what with the newly discovered Chaos cult that was their ruling class deciding to ask Papa G to send a deamon army to "free" the planet from its Imperial overlords on one side, a Splinter Fleet on the other, and Necrons attacking from a hitherto unknown side, it was agreed that it would be best to reduce everything to a thin veneer overlaying a glassed ball. The few nobles who were either unaware of the cult or refused to join, though they were usually killed off upon refusal, gathered as many people they could find and, led by lord high admiral James Scelt, boarded the few cruisers they had and began the gauntlet that led out of the xenos ridden atmosphere. Barely half survived.
The execution of the planet was short and brutal. Three rounds of twin cyclonic torpedos were launched from orbit, successfully destroying any trace that a lush planet or any form of xenos or heretic had ever been there.
After a long while of flying around anywhere they could, they were eventually drafted into assisting the Dark Angels by becoming meat shields and taking care of any job deemed beneath their time and effort, occasionally being allowed the honour of joining them on the battle field and absorbing many bullets for them.
The battle that dragged them back into the limelight as a formidable fighting force was during an attack on a cultist planet. The Chaos forces were greatly under exaggerated and there was slaughter on all fronts. The final fighters left at the end were El'Jonson himself, a chaos warlord and a dying, legless Jonathan Scelt, son of James Scelt. El'Jonson was dead on his feet and Khorne was about to be granted a Primarch skull to adorn his throne, but got hit by a skullblock in the form of Scelt armed with a bayonet meeting the the warlords ankle. As he roared in pain El'Jonson took charge of the moment and stabbed the warlord in the neck with a fallen power sword.
This act awarded the remaining forces a lot more respect in the Imperium and insured that they were allowed to gain a new planet to call their home and soon they were almost back at their peak in terms of wealth and status, though they were constantly under scrutiny by the Inquisition to stop them from any more heretical activities.
The next notable historic event they partook in was the boarding of Vengeful Spirit during the Horus Heresy where they were put under the command of a certain Ollinus mothafukin Pius in the infamous battle ( it was a miracle that they were able to fit in a ship already carrying the massive brazen ballsack Pius has ).
Half of their forces, the most elite half at that, boarded the ship, and only one man actually survived, Tiberius Scelt, the grandson of Jonathan Scelt who actually observed the legendary sacrifice of Pius and the destruction of Horus. He was carried out by the Emperor himself and in his radiance suffered only a missing left arm to show for his fight.
After the Heresy Scelt was awarded his families earlier title of Lord High Admiral and High Lord of Terran. The citizens of the Hive Cities of Onóir II live as though the exterminatus of their planet never happened, and as the two planets are in almost all respects identical this isn't hard.
The Onóir First and Foremost are a highly mobilized heavy infantry force and, though known for having an ungodly number of Valkyries and flamethrowers in their arsenal, their main weapons are their negotiation skills and the massive mounds of cash their nobility have access to. They are mainly used for boarding enemy ships, though when the chips are down on a planet, they fly in on a fleet of Valkyries playing "Ride of the Valkyries" at full blast through every speaker they have, "All Guns Blazing" is another favourite of theirs.
- James Scelt: The original bearded badass of the Scelt lineage.
- Jonathon Scelt: The one who brought them back from the brink of death.
- Tiberius Scelt: The current Lord High Admiral and leader of the First and Foremost. Badass beard owner.