The name of Pentecost will always be remembered in the history of Man. Located in the Septimus Sector of Imperium space, it was one of the few planets which wasn't scarred by the Horus heresy. Surrounded by never-ending warp storms, the inhabitants of Pentecost fell into barbarism. Cannibalism and infanticide were rife throughout the uncivilised world. The planet was beseiged in a constant state of confusion where human tribes armed with sticks and stones practiced genocide towards one another. Once the Imperium rediscovered the primitive planet, it was reclaimed in the name of the Emperor and rebuilt to be the envy of neighbouring sectors.
It was a succesful trade world. So famous was it's reputation that many alien races such as Squats, Eldar and sometimes unscrupolous Blood Axe Orks travelled to Pentecost to broker lucrative deals and trade for it's ample resourses. Pentecost became the pinnacle of Imperial excellence for many millinieums. Poverty was non-existent and the lords and labourers of Pentecost owned tremendous amounts of fillings and power. Then, one day, the world of Pentecost stopped paying it's Imperial tithes.
A minor problem, thought the Imperial administratum. The Imperium ignored the apparently non-existent payment from the rich, frontier trade-world. There were other more pressing matters. A decade passed, something was wrong for Pentecost had stopped paying their now much-needed tithes for that amount of time. Pentecost's Astropath could not be contacted. There were no reports of trade dealings with aliens for the past 9 years.
A cell of Inquisitors were sent to investigate. They discovered an empty, barren world, not a sigle microorganism could be detected by warpship scanners. Only the collosal man-made plascrete starscraper structures were left on Pentecost. What happened? Ecological disaster? Mass exodus? No, it could not be, for one or two life forms would've been found on the empty world. The planets power-grid was in perfect working order. Suspicious of the circumstances, the cell originally thought it to be a possible isolated Tyranid attack but one of the cell members, the infamous Inquisitor Brand, painstakingly managed to piece together clues, hints and scattered data records in deserted Imperial holo-libraries like a gigantic jigsaw puzzle.
Shocked, they discovered that an uprising had happened on Pentecost. Secret Chaos cults were formed by the inhabitants of Pentecost. These hidden followers of darkness recruited all psykers whenever possible and managed to influence the planet's Astropath with their dark promises of power. The small force of Adeptus Arbites, the only presence of Imperial authority on Pentecost, were easily cushed by a surprise attack from the cultists. Pentecost had betrayed the Emperor. The world was now under the domination of their new patron, The Lord Of Change, The Changer Of Ways... Tzeentch.. It was the most heinous of heresies. A conspiracy against the being of mankind..
Unbeknown to the denizens of Pentecost, Tzeentch was now as hungry as he was happy. Tzeentch ordered the inhabitants of the damned world to surrender to Tzeentch, their souls as gifts. Eager to please their infernal master, the whole planet commited mass suicide, in the form of biological self-exterminating bombs. Tzeentch's thirst for human souls, his lust and his thirst, was satisfied and quenched for that moment.
Inquisitor Brand and his cell went back to their warpship. Never again would the Pentecost's barren soil feel the touch of any living creatures..