True Form
In his true form he has more physical strength than most of his kind and he is said to have moved entire mountains of the underworld to shape his own dominion, but in the mortal world it is particularly risky for demons to resort to their true form, if it were as simple as most believe then humans would not have stood much of a chance. When a demon is slain in the mortal world while inhabiting a vessel they are cast back into the underworld but not killed, the only drawback to it is it will be so severely weakened it could take years to regain the strength to return depending on how badly they were beaten and how strong the individual. Some can return in a matter of days, some take decades but either way they live to fight another day. However once a demon resorts to their true form in the mortal world they are just as vulnerable as they are powerful, to be slain in their true form is to be destroyed permanently and it is a risk very few will easily take because even with all their strength, humans have many ways to weaken them so the risk is incredibly high.
Ability:
In an attempt to match the strength of his true form Larken has taken on the ability to absorb the life essence of living beings and convert it into raw power, unimaginable power that grants him the strength to shatter entire buildings if he took enough of it. He can steal the life essence of numerous creatures in more than one way relative to how much strength he's already obtained, the first and easiest of them is to feast on their blood but to obtain the most fearsome power requires taking the souls of living creatures. Taking the souls of animals is the equivilant of drinking a human dry but taking a human's soul is another feat entirely, the human soul is like a nuclear reactor but taking one requires an immense amount of power while in a human vessel, the only way to save up enough strength for taking a human soul would be to drain at least two dozen humans dry and then lie dormant a week to let the strength reach it's peak.
Occupation:
Larken is the head of an organization of thugs that run the underworld, like a mob of sorts, at least that's how they appear to humans too blind to see the forest. In reality he is the head of the last group of higher and lesser demons, they slowly gather their strength in order to retake the city, but unlike the last demon lord his ambitions are much greater. After they take the city he intends to opens the very gates that stand between his world and the mortal world in order to slowly seize total control of it, he would lead them and become the king of hell on earth while Lucifer reigned below.