They ate the hearts of their prey, and over years, gained their powers.
To maintain a sustainable yield, they forbade eating anything but your own kill. This resulted in a clear hierarchy within their communities.
Youngster and unskilled hunters ate mostly gnomes, elves, and other low creatures. It granted them minor magic. Enough to be useful, too little to be efficient.
The skilled and aggressive would quickly rise up as they slayed packs of werebeasts, entire drow villages, and the occasional elemental. Some were foolish enough to consume ents, they never lived long after.
The eldest and most well-fed had obtained powers across the entire known spectrum, and beyond. They would duel with gods, eyes forever on the prize, but still lacked the lifespan to see it through. Cardiac arrest, for most, as if the powers had strained their hearts too far.