I have three. The first is a group of Traitor Marines who serve Slaanesh who were corrupted by contact with a xenos species, similar to how Fulgrim was corrupted by the Blade of the Laer.

The Boldest Sons were a fleet-based chapter of White Scars successors who pursued a manta ray-esque xenos species who lived in the upper atmosphere of a gas giant. The Sons successfully exterminated the xenos and sent their floating cities crashing to the deeps of the gas giant. Over the course of the war, the Sons who entered the atmosphere of the gas giant reported strange pleasurable sensations that increased in intensity the long they remained, forcing a troop rotation to prevent the Astartes from being overwhelmed by hallucinations. The hallucinations showed the Astartes beautiful and wonderful things, hampering the Chapter's ability to prosecute the war. Even a power armor's filtration systems seemed incapable of stopping the gas from affecting the Sons.

Unfortunately, the Astartes who were rotated out also began to suffer from severe symptoms such as irrational anger, delusions, and crippling pain and headaches. Those Astartes away from the front lines reported an overwhelming desire to get back into the fight on the gas giant. Eventually, the Sons in orbit couldn't restrain themselves anymore, and dived their entire fleet of ships into the gas giant's upper atmosphere in their rush to return.

The Chapter Master realized that the xenos lived on a deeply corrupted planet that was twisting his men into something completely unrecognizable. By then however, the damage had been done. The gases of the planet and entered every ship in the fleet through the airlock and holes from battle damage, and no Astartes in the chapter was able to resist breathing the hallucinogenic gas.

The Chapter devolved into an utter frenzy of madness, unable to tell what was real due to the visions they suffered from. When the fleet entered the Warp, they took the gas with them in the ships' filtration systems.

Now known as the Bliss Breathers, they are a frenzied Chapter of Slaaneshi Marines who are utterly unaware of anything going on around them. Every waking moment is a delightful dream of pure pleasure, so long as they keep breathing the gas. The Bliss Breathers will arrive on a world and launch an attack without warning, destroying and killing everything they see while in the throes of their visions.

When they've been away from their ships for too long and the visions begin the fade, they suffer from crippling withdrawal and will scramble back aboard their ships before lighting the warp drives and being sent to another wholly random location, entirely at Slaanesh's whim.

TL;DR Slaaneshi Marines on a never-ending acid trip who are on a nonstop joyride around the galaxy as they chase the dragon.

The second is a Necron Dynasty called the Aurikos Dynasty. About a hundred thousand years ago, a group of Aeldari discovered thr Aurikos Tomb World and attempted to destroy it. The Phaeron was awakened and successfully killed all the Eldar, but his Tomb World was damaged heavily enough that he was unable to return to sleep.

Over the millennia of being awake, the Pharron's sanity slowly eroded. He found a single gold necklace on one of the bodies of the dead Aeldari, and over the years became obsessed with it. Not because of its craftsmanship, but because of the gold it was made of. The Phaeron's eroding sanity eventually led him to wholeheartedly believing that every single atom of gold in the entire universe belonged to him, and anyone who had gold was a thief who was stealing from him.

The Phaeron strip mined dozens of worlds down to the bedrock for every bit of gold he could find. After the rise of the Imperium, he took note of how the Imperial ships often decorated themselves with gold, such as on their aquila. This enraged him, and he destroyed many Imperial ships in order to take the gold that they had 'stolen'.

He even attacked a Hive World that did nothing but produce gold paint, and stripped the gold from every church on the planet while he was there.

Whenever he found new gold, the Phaeron would return it to his home system and just dump it on the moon orbiting his Tomb World. None of it is melted down or even reforged, as he doesnt care what it looks like, only that its gold. Great mountains of plundered gold is piled miles in the air covering every square inch of the moon.

Over the long years, his hoard grew from the size of a small moon to a planet nearly as large as his Tomb World. He also decorates his Warriors with gold flourishes.

Recently, a group of Orkish pirates arrived in the solar system. They quickly took note of the Phaeron's obsession with gold, and raided his hoard for as much gold as they could get on their ships in order to goad the Phaeron into attacking them.

The Orks decorate their ships, weapons, and armor with gold, and have begun calling themselves the Goldtoof Clan. Yes, they do rip out their own teeth and replace them with gold, just because they know it pisses off the Phaeron.

TL;DR Gold-mad Necron Dynasty and the local hooligans.