To sort of peel back the curtain for a moment, we've kind of drifted from that to using the Earthscorpion definition - "deathlord" is a term for ghosts that have achieved unspeakable power (at an equally immense cost) by venturing into the Labyrinth and taking a portion of the Neverborn's power into themselves. The OG deathlords are exceptional for various reasons, namely:
A) Age. They lived and died in the First Age of the world, and that means they have knowledge of things that no other creature, living or otherwise, remembers. And knowledge, when wielded correctly, is power all its own. More to the point, they have had time to reach the greatest heights of power available to a Lord of Death; any deathlord is powerful, but they're strong enough that any one of them could probably fight (or craft, or give oratory, etc.) on an even footing with a similarly-focused Unquestionable.
B) Resources. Personal power is something common to all deathlords, but the "canon" ones have temporal power - they each have other Lords of Death who have chosen to ally with or serve them, dozens of Greater Dead underlings, and great hosts of lesser minions. As it was in the era before the world was bent, infrastructure is what makes the difference between a lone potentate and a true monarch, and any lone would-be kingslayer who takes them on without understanding that will be ground to dust under the weight of nations.
C) Reputation. As powerful as they are personally, as much infrastructure as they have, the First Age Lords of Death are no more invincible than the Solar Deliberative they once served on. In theory, a force of "lesser" deathlords with sufficient time, motivation, and numbers could tear down his nation, run him to ground, and then butcher him. It would be a struggle worthy of song, the Lord of Stygia would bleed his killers for every wound they inflicted, but in the end, he would fall. The thing is, their actual power is nothing compared to that which legend and eons' worth of accomplishments give them in the minds of others. To consider taking on the First and Forsaken Lion, the Underworld's other Lords of Death first have to think of all the stories told of the Lion's prowess - that he cannot die so long as the living continue to hunger for war and conquest, that his armor is a hekatonchire which he defeated and broke to his will, how those who fall to his blade are trapped within it, to be questioned and tormented as he desires - and most of them then decide that it's a better idea to do something else. Sure, most of it sounds like total bullshit, but even under the most skeptical light it's obvious that the Lion has been kicking peoples' shit in for thousands of years, and... well, if anyone's really that strong, it would be one of them, right?