Who were/are the Giants??? What are they in myth? Vast, humanoid, frequently possessed of an attachment to a facet of nature (frost, fire, storms), frequently hostile but able to be treated with, capable of great deeds in war and peace, primal. If one includes nephilim references as well, one might consider opposition to the Creator/Allfather/What-have-you (or at least, not quite an intended part of creation.....).
Origins of all ‘Protagonist race’ sapients is from a creator or creators. Elder Races are Elves, Dwarves, &c. A gulf separates Sapients and the divine – just as the Fall of Man, but quite possibly due to Angelic Defection or Quarrels between Mortals (‘If you can’t play nicely...’) rather than the Temptation. After this fall, Elves, Dwarves, Proto-Humans were left in a somewhat weakened state. Those Angels and/or Deities that actually cared to intervene with the world were permitted to aid them. Some taught them, sheltered them and nurtured them. Others just gave them powers directly; these became the Giants. This was regarded by ‘The Top Brass’ (who or whatever that is) as giving a Child a revolver in order that they could protect themselves from playground bullies. The Angels/Deities/Aspects of the Creator responsible were reined in.
By this point, of course, the Giants had become what they were: able to weather cold, hunger and other forms of adversity with impunity (they were able to walk through the Arctic naked, but most wore clothes as a matter of etiquette and beauty). Capable of building tall towers and great cities; they showed great strength with their arms. However, being aware (or made abruptly aware by the Powers that be) of their potential to disrupt the development of the other races, they sequestered themselves in inhospitable areas (partially out of pride, it must be said: ‘Look, humans! Look at my palace! It’s balanced on top of Mount Everest!’). They developed swiftly, breeding herds of beasts to an appropriate size for themselves and raising up vast cities.
Those that died, after many centuries of life, lingered: they were made to survive this world, and this impeded somewhat their possible re-uniting with the Divine. A certain amount of ancestor worship sprung up, and those spirits that lingered long enough took on roles of importance – at least until they could work out the method to ascend. Those spirits that gave good advice (or at least plausible advice) got more out of it than those that didn’t; thus, few of the ancestor spirits that received worship were absolute bastards. Some of the Spirits worked out the Clerical Spells function to make themselves even more into Godlings – but those that abuse in get their fun shut down by the ‘Top Brass’.
They developed swifter than other races, and whilst they didn’t have necessarily more inherent problems than any other race, those problems became threatening on a wider scale quicker, and the cities of the Giants were cast down (Weapons of Mass Destruction? Perhaps.). Those that remained on what would become the Trygvir Isles lost much of their former prowess, for all the attempted guidance of the ancestor spirits. Of those giants killed in the destruction of the cities, many became unquiet, tumultuous and increasingly incomprehensible spirits, minds ravaged by the drama of the fall and the ages of half-life and desolate ruination thereafter (whereupon many take on qualities similar to the spirits and giants of Tim Powers’s Declare and The Stress of Her Regard; IE, attaching themselves to old statues of themselves or to desirable persons or of the ruins of their old house). The ruins of the Cities are not a safe place to go. None but the bold visit them.