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 Obscuris, 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. 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, laid siege on the Fang, successfully broke in, destroyed a geneseed mutation cure, killed whole bunch of awesome guys including the current Great Wolf (and almost killed Bjorn), and then teleported away avoiding any major payback (though Space Wolves somehow decided they won), and second during the Age of Apostacy by the mad cardinal Buharis, who failed miserably despite having a 10000 to 1 advantage in manpower. Finally it was invaded by combined force of Ordo Malleus, Grey Knights and a Space Marine chapter during which immense badassery insued as Logan 1) Killed a Grandmaster and some Justicars and 2) Killed the Inquisitor trying to fuck with him. Bjorn convinced him to negotiate a cease fire because there was more cheese than his space furries could handle. Sense 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.