Ok, this is what I managed to understand do far: this is an ABO world and MC is a transmigrator. The crown prince wants to be sure his brother the 5th prince is a bêta and not a threat. He asked the prime minister to marry his eldest omega son to 5th prince and force his heat to make it impossible for the alpha to keep hiding his nature, even if it means the omega will die without an alpha during the estrus. The original owner of the body did die and the "president" appears. The 5th prince is really an alpha and reacted to the omega but manages to restrain himself and goes away, promising to the omega to take care of his body after his wasteful death. So MC has been sacrificed by crown prince, father, brother and husband in a struggle for power.... Lovely!