I want to know if there is any reference in our puranas saying that Duryodhana is awarded as leader of Rakshashas by Lord Shiva. The reason for his birth is a boon to Rakshashas or something along that line.
I am doing research of Mahabharata in parallel to asking this question and I would like to add my findings here as an answer here for future reference
When Duryodhana goes to Aranya where Dharma raja and his brothers are residing he gets into fight with Gandharvas and is defeated by them. Gandharvas capture Duryodhana and soldiers of Duryodhana will ask help of Dharma raja to save Duyodhana.
Dharma raja then sends his brothers to bring Duryodhana back from Gandharavas and they obliged.
Having been saved by Dharma raja and his brothers, Duryodhana decides to sacrifice his life by doing Prayopadesha. At this very moment Rakshashas will take him to Patala loka using magic and they will speak the following lines to Duryodhana
'O Suyodhana, O great king? O perpetuator of the race of Bharata, thou art ever surrounded by heroes and illustrious men. Why hast thou, then, undertaken to do such a rash act as the vow of starvation? The suicide ever sinketh into hell and becometh the subject of calumnious speech. Nor do intelligent persons like thee ever set their hands to acts that are sinful and opposed to their best interests and striking at the very root of their purposes. Restrain this resolve of thine, therefore, O king, which is destructive of morality, profit, and happiness, of fame, prowess, and energy, and which enhanceth the joy of foes O exalted king, know the truth, the celestial origin of thy soul, and the maker of thy body, and then summon thou patience to thy aid. In days of old. O king, we have obtained thee, by ascetic austerities from Maheswara. The upper part of thy body is wholly made of an assemblage of Vajras, and is, therefore, invulnerable to weapons of every description, O sinless one. The lower part of thy body, capable of captivating the female heart by its comeliness was made of flowers by the goddess herself--the wife of Mahadeva. Thy body is thus, O best of kings, the creation of Maheswara himself and his goddess.
In summary Rakshashas earned Duryodhana as their leader by doing austerities and receiving the blessings of Mahadeva.
Source: Vana parva:Ghosha yatra parva