In the third edition Dungeons and Dragons cosmology, Cania is the eighth level of the Nine Hells of Baator, the plane of Lawful Evil. It is situated between the red wasteland of Maladomini and the hellish depths of Nessus, and is ruled by the archdevil Mephistopheles. Mephistopheles (never call him Mephisto, he has tortured angels for millennia for less than that) is one of the most powerful of all Baatezu.
Cania itself is a bleak, inhospitable, utterly desolate frozen wasteland with rivers of magma. (This is quite consistent, believe it or not: Mephistopheles is a study in contradictions). Even these magma rivers provide no respite from the horrible winter chill that permeates the entire layer. Anyone, other than a Baatezu or most Tanar'ri, unlucky enough to end up here while still alive will freeze to death in an hour or two without some way of starting a fire. Because of the extreme chill, only magic can start fires reliably here.
This chill sounds somewhat more inviting than the steel-melting flames of Phlegethos, but I assure you, it is worse. The pain of flames, you see, lasts but a mere few seconds before the poor traveller is reduced to ash. Cania's chill, though, never leaves your bones. Here on earth, try taking a walk around in Antarctica during the deep of winter for a glimpse of what Cania might be like.
What of those tortured souls damned for all eternity in Cania? Well, the watchword is despair. Utter, soul-crushing, mind-destroying despair. Most of those, living and dead, who come to Cania do so involuntarily. They wander, cold and alone across the semi-infinite plains of frost until they freeze to death or are slain by marauding Gelugons and Cornugons. Those who escape this fate frequently become so mired in despair that they hurl themselves into the magma rivers. No few of them find themselves awakening as lemures, still in Cania, only now bound in slavery to Mephistopheles. Barring magic, Cania is damnably hard to escape. And even if you do, physical escape is only possible to either Maladomini or Nessus, or via the river Styx to the eternal battles of Acheron or the hope-eating Grey Wastes of Hades. Verily, then, Cania is a place best avoided unless you are an experienced planar traveller.
What if you are experienced, you ask? Well, a few encampments of devils and travellers exist, most near the ice mines. Most of those who voluntarily travel here are Tieflings, Inevitables, Githzerai and Githyanki. A few Slaadi show up here, too, though the plane's lawful nature pains them greatly. Some frost giants and ice trolls dwell here natively, though even they fear the devils.
Information taken from the 3.5e Dungeon Master's Guide, the Manual of the Planes, Book of Vile Darkness and the game Neverwinter Nights: Hordes of the Underdark, with a not inconsiderable amount of GM's creative license taken.