In its living form
Niddhoggr was a massive, 320 ft long hellkite of monstrous power. When alive he was settled in the highest volcanic mountain of Drake valley, regularly slumbering in its molten pool. When it awoken it was said that the very region itself would shake and trigger a volcanic eruption. The dragon was infamous for its brute strength that came with its size as well as the intense flames that it could create, far more potent then the typical dragon fire as it could melt stone to lava easily. The dragon died in combat with another monstrous Dragon, Jormungand, the great sea Wurm. A sea dragon that's size could stretch along an entire coastline. The battle was a close brawl, Jormungand barely survived, but like all savage beasts it happily consumed the corpse of the once great dragon.
Initially, Niddhoggr was reluctant to join the druids of drake valley, sometimes nicknamed the dragonspeakers or dragon shamans. He had no interest in serving man, and while he wasn't necessarily evil, he had no benevolent intentions either despite being exceptionally intelligent and able to speak. William's father, Drake, was the one to convince Niddhoggr to join their rite. Convincing the great dragon that his great power can be used not only for vengeance, but to be in general, used once more in the world. The spirit accepted on the condition that his chosen patron, who would be Drake's son, one day slay Jormungand. To their aggreement, the dragon contracted with Drake who was at the time still an infant.