Fenris

"There are no wolves on Fenris."

--Popular saying during the Great Crusade.

Even the tyranids would need a blanket here.

Fenris is a frozen little hellhole in Warhammer 40,000, most notable for being the home world of the Space Wolves. Named after the ancient Viking wolf monster involved in Ragnarök (because the Space Wolves are also Space Vikings, at least at first glance), Fenris is located in either the Segmentum Solar or the Segmentum Obscurus. The Imperium in all politeness, classifies it as a Death World, meaning that everything in the planet's biosphere is lethal and will always try to kill you and most of the times succeed. This makes it the ideal place to recruit for an army of space vikings.

Fenris is composed of a single continent called Asaheim (Literally translating to Home of Gods in old Norse), with hundreds of islands in its vast world sea. Those parts of Fenris which aren't completely frozen over can only sustain human life for no more than a few minutes, and alternate between being frozen and undergoing earthquakes. The Fenrisian wolves are among the most dangerous predators in the galaxy, as they can reach the size of a full-grown horse. The dominant sea life are octopi-like monsters called Kraken, and they have tentacles that can reach miles in length -- some paranoid types have claimed that these may actually be stranded, devolved remnants of a past Tyranid invasion, just as they claim is the case with the notorious Catachan Devils. There are also bipedal, anthropomorphic... things ...walking about the snow of Asaheim that the locals refer to as trolls, as well as the Wulfen- abortive Space Wolf neophytes that have devolved into wolf-like monsters. All in all, it's one of the deadliest planets in the galaxy.

And the Space Wolves love every minute of it.

It has been invaded only three times:

• First in M33 by the Thousand Sons and Magnus himself, who lured most of the chapter away, brought a million-numbered army of mortal cannon fodder, laid siege on the Fang, successfully broke in, destroyed a Wulfen curse cure, killed a whole bunch of awesome guys including the current Great Wolf and almost including Björn, and then teleported away with his Thousand Sons and their ships, avoiding any major loses (aside from that massive army of mortals, which he left behind as a distraction). Space Wolves somehow decided they won. (Mostly because the Leader of the Wolves REALLY REALLY hurt Magnus. Space wolf style! Basically he beat Magnus down really hardly after falling off the massive mountain that is the fang, Punching him into a wall, ripping off his skin revealing a humanoid cloud of light, making Magnus yell out in pain and almost ripping out Magnus's eye. Magnus recovered in a spectacular way though. That being catching the Wolf lords first then literally annihilating his soul with a flick of his wrist. THOUSAND SONS FOR THE WIN FURRY FUCKS!!!!)
• Second one was during the Age of Apostasy by the mad cardinal Bucharis, who failed miserably despite having a 10,000 to 1 advantage in manpower. He actually managed to lay siege on the Fang but failed to do anything to it (unsurprisingly, given TS needed custom strike cruiser turned into a giant plasma cannon to break through the fortress's void shields), until the companies, who where stationed on their ships when his fleet arrived made a planetfall behind his army and sandwiched it between themselves and the defenders of the Fang, massacring it to a man.
• Finally it was invaded by combined force of Ordo Malleus, Grey Knights and a Space Marine chapter during which immense badassery ensued as Logan Grimnar 1) Killed a Grandmaster and some Justicars and 2) Killed the Inquisitor trying to fuck with him. Bjorn the Fell Handed convinced him to negotiate a cease fire because there was more cheese than his space furries could handle. Since then the Inquisition has avoided Fenris.

All three times the harsh environment of Fenris scored more kills than the Space Wolves, which is something to say when you are competing with more than a thousand viking superhuman killing machines.

Round 4 aka Yiff in Hell

Recently Fenris has been attacked again by Chaos forces and GW has really made it clear things are going bad for them, with the Dark Angels severely damaging at least one planet in the system, Midgardia.

Then Magnus the Red, Daemon Primarch of Tzeentch shows up. Yeah, things are going really well for the Wolves.

Magnus succeeds at wiping pretty much all life on Fenris in the most disturbing and cruel way, but ultimately gets banished for his trouble. Then the Inquisition and Grey Knights proceed to wipe the last surviving Fenrisians, because they're tainted by Chaos (and also saw daemons and GKs), and this time SWs have not enough strength to stop them. So Wolves are now living on borrowed time, being effectively doomed to either die out, mutate into wulfen, or lose their sanity and turn to Chaos like the Wolf Brothers before. If only they had that cure Strawberry destroyed the first time he attacked... At least the space yiffs would get their chance to go down in a blaze of glory, seeing as the 13th Black Crusade starts pretty much immediately after the 4th battle for Fenris.