Land of ice and perpetual cold. Though little is known about it from either myth or testimony of travelers. The land is said to be cloaked in a perpetual mist so thick that most days one would struggle to view a hand held in front of a face. Dark tales also speak of the mist being poisonous to any who venture within. In any case between the bone chilling cold, poor visibility and possible poison very few who venture into this land have ever returned.
So hostile to life are the frozen wastes that even in ages long past the gods did not venture far into the world most struggling to get as far as the citadel of Bölþorn an outpost of the frost Jotun in the relatively mild region closest to Hel. No one is ever documented to have ventured further in and returned alive save perhaps for Odin who claims to have traveled Niflheim in his youth, but as ever the Allfather has hoarded his wisdom on the subject, only warning against other attempts to brave that frozen realm.
The inherent hazardous nature of travel in Niflheim has made an accurate knowledge of it's inner geography hard to come by. Ancient fables mention that at it's centre lies the well of cold called Hvergelmir. From this well are said to flow nine rivers called Svöl, Gunnthrá, Fjörm, Fimbulthul, Slídr and Hríd, Sylgr and Ylgr, Víd, Leiptr and Gjöll.
Virtually all travellers to Niflheim travel in the relatively temperate regions close to the borders of hell and far from Hvergelmir itself. Through this region flows the rivers Víd, Leiptr and Gjöll and tales talk of hardy crops and animals being cultivated by the natives.
The citadel of Bolborn the only known ice giant settlement of significant size is said to be somewhere along the length of the river Vid.
Due to the fact that the very world in which they live proves deadly to others little detail in known about the culture of the being who dwell within the cold reaches of Niflheim. Although referred to as ice Jotun there exact relationship to those that dwell in Jotunheim is unknown.
Whilst in his cups Thor claims to have fought ice giants from Niflheim on several occasions describing them as rimed in ice with cold blue eyes but volunteering little information beyond this.
OC Note: the information presented below is IC - you may choose to believe as much or as little of it as you wish
An Excerpt from : Nordic Myths Truth or Fiction
The creation of the nine worlds is one of the less well known Nordic Myths and so it should come as little surprise that it is tied to two of the most mysterious worlds namely Niflheim and Muspellheim.
According to the myths at the beginning of time there was just the great empty void Ginnungagap. The first world to exist in the void was Muspellheim the world of primordial fire which awoke in the south of Ginnungagap. This world existed by itself for uncountable ages before far to the North another world came into being. This was Niflheim the land of primordial ice, cloaked in poisonous mists. At the centre of Niflheim was the well a well called Hvergelmir in later ages from which flowed nine frozen rivers. These rivers flowed out into the void but as they did so they came too close to the fires of Muspellheim and began to thaw.
The liquid from the rivers fell into the shape of a man, he was named Ymir and known among the jotun as Aurgelmir. From his own body Ymir is said to have birthed three children, a male and female the first of the Jotun and a six headed monstrosity. His first two children bred creating the race of the Jotun. For many generations this new people lived in the temperate middle regions of Ginnungagap however they found Niflheim and Muspellheim themselves too inhospitable and unwelcoming. Soon there was a thriving civilization presided over by Ymir but over time the first of the giants became corrupt and tyrannical until his people despaired at his unjust rule.
This all changed with the coming of Borr Son of Buri. Borr married Bestla who bore him three sons Odin, Vili, Ve. These three grew in age and power eventually waging war upon Ymir and slaying him. With the deathblow struck however Ymir's lifeblood poured out in a seemingly endless torrent Drowning all who dwelt in Ginnungagap bar two jotun and flooding even the far reaches of Niflheim and Muspellheim.
From this chaos Odin and his brothers then set about forming the nine worlds from Ymir's titanic corpse. His skull became the sky atop which rested Asgard and the upper realms. Beneath the Dome of his Skull was Midgard whilst the realm in which Ymir had dwelt became Hel. From what remained of Ymirs body grew the world tree Yggdrasil which would eventually expand to connect all the worlds. Thus it is that the Jotun have great power over things which grow from the earth as they are formed from the flesh of the earth itself. Finally Odin plucked a spark from Muspellheim placing it high in the sky to form the sun, whilst the moon was formed from a shard of Niflheim to maintain the balance in the worlds.
Thus ends the tale of the creation of the nine worlds. This myth is reportedly told to a Nordic King by Odin himself. Which I suppose is not that surprising when one considers quite how few survived the events preceding the creation of the worlds.