Pit Fiend Lord of Avernus, the First plane of the Nine Hells
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Invited onto the Prime Material by Hildraft and (unsuccessfuly) bargained with for a betrayal of his vassal Zerhal Avara.
The most successful achievement of Bel was the trick known as the Four-Cross. During intense fighting, the pit fiend general suddenly ordered his troops to switch sides, and they slaughtered the other devils on the field. The demons, naturally, were leery that the pit fiend would truly betray his own. Bel tried to win their trust by revealing diabolic battle plans.
The pit fiend even concocted a scheme to further double-cross the devils: He should return to the Hells, pretend to rejoin his fellows, and then betray them again on the battlefield. The demons agreed, and Bel did indeed return to his home plane. But unknown to the demons, he turned on his new comrades by giving the devils rough details of demon plans and troop sizes.
When next the forces of Hell engaged those of the Abyss, Bel rode with the devils, but he switched sides again in the middle of the fighting - exactly as he told the demons he would. This time, Bel and his troops fought on the side of chaos for a year and a day, thus earning the full trust of the demon lords.
And then the pit fiend led the Abyssal high-ups into a long-arranged diabolic ambush, crippling the demonic efforts for a good decade - they were reduced to fighting a defensive battle from the Abyss. Bel was rewarded for his elaborate double-double-cross with command of the armies of Avernus, the first layer of the Nine Hells; his first step towards its' rulership.