Asmodeus

Asmodeus is a supreme ruler of the Nine Hells and one of the oldest gods among The Grim Realms. He used to interact with mortals more frequently and offer them a bargaining of power, wealth, or knowledge in exchange for their soul. He does not gain power by worshippers, and instead utilizes the power of a raw soul to heal his wounds.

Origins
Asmodeus was created before many other deities and before the material plane was inhabited by capable mortals. He was born alongside Jazirian in a struggle for power. The two of them has opposing ideals but each stood for Law. When the two had fought through the planes at a time before planar barriers, their fight had caused them both to fall into the plane of Baator. Jazirian, having wings, flew away to avoid damage. Asmodeus however fell into Baator and created what we now know as the Nine Hells. He remains at the deepest and most sunken pit in Baator, using lesser creature's souls to heal his wounds and fuel his regeneration. Once strong enough, his plan is not clear.

The main disagreement was the way in which they wanted to deal with the growing maw of the Abyss plane. Full of demons and ever dangerous and threatening, the Abyss grew and absorbed parts of other planes. Planar barriers were attempted to be constructed but the Abyss plane found new ways to leak into others. Asmodeus was willing to use power in any way he could to stop the growth and influence, whereas Jazirian believed there must be a way to stop it without sacrifice.

After his fall, Asmodeus, desperate for strength used his magical eyes and astral projection to look into a source of power and aid. He found the material plane beginning to grow with life and creation and more importantly: souls. Each mortal life had a soul, a form of Essence, which could be utilized by him. He bargained with the mortals and told them stories of the growing abyss and offered them power and glory in exchange for their soul. They were given power and transformed into devils. His army of devils fought viciously with the demons of the Abyss, but never truly winning, only staying the Abyss' growth. As other deities began to be born and the situation of the Abyss was noted, it was generally agreed that a pact must be formed to maintain the planes.

Ultimus Pact
A desperate pact was created to aid the fight against the Abyss, Asmodeus needed mortal souls to create devils for his army, and the other deities understood this. They had a meeting in Shadowfell argued over what they were willing to contribute. After many long years of disagreement, a final agreement was made:

Mortal souls would be sent to Shadowfell, they would be judged by The Raven Queen, and those found worthy could be taken by their deity to wherever they may choose. Those who had bargained away their soul or had not lived according to the standards of their deity are given to the Nine Hells and ultimately, Asmodeus for his army. Of course, Asmodeus neglected to mention to the others that the figure who appeared before them at the meeting was a mere astral projection of himself in the form of a devil. Asmodeus eventually decided against doing business with mortals and other gods directly and began the process of creating his 3 avatars whom he could delegate his work to.

Avatars
He created 3 avatars of hell, dubbed "The Prime Devils."

He broken part of his tail from his body and used parts of it to create 3 creatures of giant size and power.

From the bulk of his muscle, he create Lucifer, a hulking reptilian creature with boney horns and massive strength.

From the bulk of his skin, he created Baal, a resilient quadrapedal mass of stretched skin and muscle with crab-like legs and regenerative properties.

From the bulk of the bone, he created Mephistopheles, a skeletal entity with large spires of bone and a strong grasp of arcane ability.

Each avatar was given a different set of instructions, but all goals were similar: they were to use their given strengths to offer to mortals to lure them into a contract that would forfiet their soul.