[HoneyBadger]

Niefelheim is (kinda/sorta) the name of the place where the Niefel giants live/originated. But, it's really more of an idea, since Niefelheim is the place of endless cold, in Norse mythology. "Son of Niefel" or Nifel, if you like, is suggesting that the Pretender is the spiritual descendant, or inhabitant once-removed, from that place. Like a "Son" of Britain, or an "American Son".

There's no real embodimental ruler of Niefelheim in Norse mythology, that I'm aware of, although Muspelheim, it's hot counterpart, was ruled by one of the largest and most dangerous giants, Surtr.

Ymir would be the actual father of the Jotun race-which includes the Niefel giants, the latter-day Jotuns, the Muspel (fire) giants, the mountain giants, and even the sea giants (for there were such things), as well as being the father-through the line of Thrudgelmir-of the Troll race. Ymir may or may not have been the grandfather of the Aesir, that's debateable, but the Jotun race was certainly able to interbreed with the Aesir.

Ymir was so big that his corpse was used to create the very stuff of the world, and from his corpse was spawned the Dwarf race, transformed from maggots, and the Human race, created from trees. So yeah, Ymir was biiiiig.

Labling a Jotun as, say, a "mountain" or "sea" giant is dangerous though, because they varied greatly in power and nature, and while some would qualify as, say, an "ice giant", they were every bit as complex as humans, and even more so, being an ancient, magically powerful, and ultimately alien, unpredictable, mysterious and protean race.

Some Jotuns were themselves as large as mountain-ranges, but they could also be human-sized, or atleast Aesir-sized, and they ranged from extremely hideous (tusks, multiple limbs-Thrudgelmir had 6 heads, etc.), to quite beautiful-the wives of the Aesir. They could also commonly change shape, into not only eagles, but otters and wolves. Maybe not all of them, but certainly a good portion of the population.

This was exemplified to the greatest degree in Loki, who was half (muspel) giant, half Aesir, and could change his shape into anything, male or female, and even give birth.