Obviously, there is little room for background material in one page adventures. And I didn't want to be too direct about this point in the module. However, the full background of the Ironbones is the following.
This is your personal SPOILER ALERT.
Ironbones is a Stone Giant who lived with a clan of 11 or so other giants in the natural cave complex inside Fogbreath Peak. Ironbones was one of the weaker males in the clan and was often ridiculed for it. Perhaps this is what drew him to seek out the company of bears, with whom he found mutual rapport.
One day, while sulkily foraging in a remote place, Ironbones came across the suppressed and evil tome The Triumph of Worms, which is part magical instruction and part inculcation into the worship of darker powers.
Over the course of many months, Ironbones read and re-read the book, becoming more reclusive from his clan. He showed a talent for dweomercraft, which is rare aptitude in any giant.
Setting his trap carefully, Ironbones caused many of the male members of his clan to be away from the caverns. He then viciously slaughtered all the females and few giantlings who remained behind. After the deed was accomplished, Ironbones quickly fortified the cavern with magic, ramparts and traps that severely weakened the returning males. With the help of his hireling, Joey the Orge, Ironbones slew all but the chieftain of his clan, who affected a reluctant escape.
All that was left was to build a new alchemical laboratory in the unused parts of the cavern, instruct his ogre to improve the security of the lair and unfold the remaining secrets of The Triumph of Worms in peace.
Until you meddling humans and demihumans showed up...