FYI, this is a first draft.

Nebiros, is the Abyss's newest Demon Lord, haven fought his way up from a lowly demon slave, to the highest echelon's of demon society. He realms Sheoful, a hellish volcanic wasteland of fire, brimstone, lakes of fire and lava, and red skies. The layer is so hot, that it rivals the Plane of Fire on the Elemental Plane. It is also an obscure layer of the Abyss that never paid much attention to the mortal plane, nor got much attention from the other layers. This has been the layers greatest strength and weakness.

In terms of strength, it has avoided most of the fighting that occurs between the other layers, though violence between demons residing in Sheoful is as common in the other layers. This has allowed the layer to create a vast army of demons who are ready to pounce at an unexpected moment.

In terms of weakness, the infighting of demons in the layer itself means it's armies are not accustomed to fighting the armies of the other layers and have never launched any invasion of the mortal realm. And with it's previous Demon Lord's isolation policies, no one on the mortal plane knows of Sheoful. No one fears it. It's quantity of claimed souls are among the lowest in the Abyss, and most of it's demons are born from the Abyss itself, rather then from claimed souls.

In other words, Sheoful is the Abyss equivalent of a the Shire. It plays little importance in the overall war games of the Abyss, and is often overlooked.

That all changed when Nebiros killed the previous Demon Lord and took over the layer. He decided that from now on, Sheoful would no longer hide in the shadows and be obscure. Now it would solidify it's place in the cosmos. And having little interest in other layers, Nebiros set his sights on a greater prize, the mortal plane.

In spite of his ambition and lust for conquest and power, Nebiros was still pragmatic enough to know that his armies lack of experience in fighting non native foes was a problem. So, to even the odds, he scanned the mortal world, and struck a deal with a newly arisen Vampire lord for an alliance where they would divide the mortal realm among themselves. To increase odds, the Vampire Lord offered the idea of a larger alliance then once thought, to which the Demon Lord accepted. Soon, they recruited a Drow Queen, and Hobgoblin Emperor into their alliance. And they became known as the Dark Alliance. Since Nebiros and his Demons are confined to the Abyss and can only send limited influence and Demons to the mortal plane, these "Allies" are also meant to open portals for him and his armies.

Nebiros himself, is a hardcore social Darwinist who believes in survival of the fittest. The strong are meant to rule and slaughter, while the weak exist to serve or be slain. This drives his ambition because he believes that his layer has grown too fat and lazy fighting among itself. And Nebiros despises complacency.

In spite of his ambition, he is pragmatic enough to know that the strength of his layer and armies is a work in progress. And the start of his scanning of the mortal plane means that he has little intel on how mortal armies This is why he recruited that Vampire Lord, Drow Queen, and Hobgoblin Emperor into his "Dark Alliance". He needed to see how their armies of undead, Drow, and Hobgobling fight against the defending armies and what they are capable of, so he can create effective strategies. But like all Demon Lords, he has no care for his "allies" and intends on backstabbing them all once they have outlived their usefulness. Then he can unleash his armies on the first of hopefully many invasions. Speaking of which...

Nebiros knows that in spite of all his plans, there is a chance this invasion could fail. In fact, winning is more of a bonus objective to Nebiros. What Nebiros really wants, is for the mortal plane to know his name, and fear him. And regardless of victory or defeat, his armies will become bloodied and experienced, and more likely to succeed in a future invasion. And he knows that if he is feared, cults and worshipers of him will arise, allowing for options for creating portals for future invasions. After the PC's defeat him, right before he's cast back into the Abyss, he lets out this line.

"I'm impressed that you managed to defeat me. But this was never truly about victory or defeat. For now, this wretched world knows the name 'Nebiros', and it fears it, and will not forget it. I will be the fiend that haunts your nightmares, and the nightmares of your descendants. The evil that mortals will either fear to speak of, or seek to appease. You won the battle... but I've won the war."

They seem fine to me but  could use some personality and mechanical fleshing out.  It's very important to how they actually interact with other characters. 

As an example 

There is a bit of a theme of inferiority and wanting to be noticed/ prove themselves which is a pretty common motivation for a villain.  If you wanted to build on that you could incorporate that into his design with things that either present themselves as more than they are or are not noticed.  So like 

• A puffer fish like design that's big and buff with toxic spines - Poison and damage on hit mechanics. 
• A shadow demon - hide an teleport mechanics
• A camouflaged chameleon - hiding and shape shifting mechanics 

As for personality based on that description they sound like a bit of a narcissist that thinks they're unfairly ignored so they may be enraged when attention is not on them and try to buff themselves up so subplots like 

• Attacking other demon cults out of jealously 
• Challenging the party to a duel to prove their might ( and then cheating) 
• Being enraged when people belittle or out shine him 

The kind of character that will declare victory after losing probably also likes to talk themselves up allot. 

that survival fo the fittest thing is already used by two demon lords, but other than that this is great. maybe give him some trait that makes him stronger against beings stronger than himself to signify his rising in the ranks