Dullahan was a creature that was considered unique in the Redredane Realm. He could be categorized as a weak creature but also strong at the same time. That was because his strengths bring weaknesses but also advantages.
The source of its power came from the souls of living creatures. The more souls he absorbed, the stronger and more Mana he had. But that had a downside, as there were requirements in choosing which souls he could enthrall.
Living creatures with the blessing of Mana were the limitations. No matter how much stronger he became, he would never be able to absorb the souls of living creatures that had Mana.
That was also what caused Dullahan's power not to develop, and he failed to become one of the leading pioneers in the Great Warlock's army, Ne'ro. If only he didn't have that limitation, it wasn't impossible that he would be on the same level as the Seven Generals or the Troll Warlord, Val'gard.