Fall of Shadowbrink
"Never interrupt your enemy when he is making a mistake."
- – Napoleon Bonaparte
The fall of the planet Shadowbrink demonstrates the first time that Games Workshop actually allowed the Tyranids to win a fight, rather than being defeated at the last moment by the surprise appearance of a named character with plot armor or a miracle mcguffin device. It focuses on an Imperial owned world named Shadowbrink which was put on the special order menu by the Tyranid Hive Mind, but there was a secret which only the Governor and the Inquisition knew about that went as well as expected.
In late M41, a tendril of Hive Fleet Leviathan set its all devouring sights upon the planet of Shadowbrink. The usual response after seeing this coming, with the exception of having more guns than Cadia, is to evacuate both the planet and your bowels and leaving the former to the swarm to pick apart like a roast chicken. This time however, the planetary Governor refused to evacuate the planet and instead told everyone to dig in, buckle up and get ready for a party.
The real unknown reason for this was because there was some ancient device from the Dark Age of Technology deep underground. This was constantly guarded by a permanent detachment of Grey Knights and Librarians. It was essentially a Stargate type device, but all they knew was that it held some Alpha Level warp fuckery inside, and so the Imperium were trying to work it out and shut it down before something bad happened. Librarians worked tirelessly to shut it down for good, but they ran out of time and it started turning on...
Eventually the fleet arrived and wiped out all orbital defenses within about 3 hours, a Sci Fi version of what happened when France got invaded in 1940. With nothing stopping them and dinner bells ringing, the Tyranids made planet fall and started eating everything in sight.
With the population outnumbered 20 to 1, all major cities began to fall as they got closer to the beacon broadcasting from the device. The Grey Knights themselves were forced to commit to the fight, but only ended up delaying the tide like a fly hitting a windshield. While all this was happening, the device began to change. All the bloodshed and terror caused by the all-you-can-eat buffet above was charging the gate, resulting in a warp gate flying open and unleashing a whole host of daemons ready to get a fight started of their own.
The host was led by the infamous Quadrifold Abominatum, a four man boyband of badassery that have been kicking ass and taking names for millennia. All of them were Greater Daemons. Out came The Great Unclean One Shub'Luth'Gug, the Bloodthirster, Hak'Vasha, the Lord of Change K'rix'xi'kra, and the Keeper of Secrets Lesh'Jae'Thi'Hah, ready to fuck shit up.
Since daemons don't really do much for the Tyranids as they can't eat them, Tyranids have always ignored them. This time however, they were forced to confront them as they were so mad that someone had beat them to the dinner table and were raging to fight the first thing in sight. This led to a huge bust up between the Hive Fleet forces and the Daemons.
With the daemons being notorious for kicking ass, and the Tyranids famous for adapting to things and changing their fighting techniques and even their biology to defeat threats, the fight went on and on until reaching a stalemate. A stalemate that was not in the daemons' favor, because the Chaos gods were getting bored. Especially Khorne, who was craving real blood that day, not "worthless alien ichor." One of the Greater Daemons grew some balls and decided to finish this by leading a charge at the biggest Nid he could see. Unfortunately he was ambushed by Zoanthropes who used their psychic null effect to one-shot him back into the warp.
Pissed off by this, his colleagues decided to avenge his banishment... except for the Lord of Change K'rix'xi'kra. He claimed "fuck this shit" and ran back to the portal. After this, there was nothing stopping the Tyranids from steamrolling the daemons back towards their portal, the bloodthirster being dragged kicking and screaming, throwing a hissy fit that all that expended energy and rage achieved nothing.
Indifferent, and slightly bewildered, the Tyranids then decided to have dessert, finish up and then move onto the next planet. The Tyranids replenished their losses by absorbing the dead, the daemons just went back to the warp, and the planets original inhabitants were completely wiped out, so I suppose the biggest losers out of this was the Imperium... Awkward.