Sounds about right. The souls of those who were, while not champions, great warriors in their own right.
Although if Greater Spawn (the demonic golem-esque variety encountered in the underworld,) can also be encountered either alone or in very small groups in the furthest caverns, those hearts can be used to undergo a sort of transformation.
It is said that Greater Spawn house the tormented souls of the greatest heroes of uncounted dwarven holds. Should by some miracle the monsters be slain and their hearts taken to the gods, an option is given: To be granted a suit of armor of that most treasured mineral, or to become one with the spirit, allowing their devotion to the dwarven people and their unwavering might to return to the mortal world through a sort of symbiotic possession.
The dwarf who offers to take on this champion is fundamentally altered, being now possessed of a second soul while becoming impossibly strong and agile and granting mastery of marksmanship, melee combat, the use of armor and shield, and the use of axes, hammers, swords, maces, and spears, as well as divine protection against the ravages of time and disease, that they may serve their people for as long as good fortune permits.
These Einherjar, as they are called, are few and far between, as becoming one entails battling the mightiest servants of Holistic Detective and claiming victory with their black hearts, though they are sometimes seen. And however mighty they may still be slain by mundane causes, such as dehydration, starvation, industrial accidents, or poor fortune in battle.
The trade off: Suit of adamantine armor without needing to mine it, or an unaging syndrome-immune super soldier (who still needs to be equipped which depending on the skills of your armorers and weaponsmiths may or may not be an issue.) Probably also prone to rage and utterly fearless, which may make them difficult to control.