Fenris

"There are no wolves on Fenris."

--Popular rumor during the Great Crusade

Fenris is a frozen little hellhole in Warhammer 40,000, most notable for being the homeworld of the Space Wolves. Named after the ancient Viking wolf monster involved in Ragnarok (because the Space Wolves are also Space Vikings, at least at first), Fenris is located in either the Segmentum Solar or the Segmentum Obscurus, and is what the Imperium politely classifies as a Death World, meaning everything on the planet is trying to kill its human inhabitants. Which makes it the ideal place to recruit an army of space vikings.

Fenris is composed of a single continent called Asaheim, with hundreds of islands in its vast world sea. Those parts of Fenris which aren't completely frozen over and unable to sustain human life more than a few minutes alternate between frozen and undergoing constant earthquakes. The Fenrisian wolves are amongst the most dangerous predators in the galaxy, and they reach the size of motorcycles SMALL HORSES OR APCs! The dominant sea life are octopus things called krakens, and they have tentacles that can reach miles in length. There are also bipedal, anthropomorphic... things ...walking about the snow of Asaheim that the locals refer to as trolls, as well as the Wulfen- failed Space Wolf neophytes that devolved into wolflike monsters. All in all, it's one of the deadliest planets in the galaxy.

And the Space Wolves love every minute of it.

It was invaded only thrice:

• 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 vulfen curse cure, killed whole bunch of awesome guys including the current Great Wolf and almost including Bjorn, 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.
• Second one was during the Age of Apostasy by the mad cardinal Bucharis, who failed miserably despite having a 10000 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.