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The 2nd candidate to the throne....
Spoiler
As i read from the novel, he is not a royal blood but instead made the beloved lover and youngest son of the emperor hostage. I think the house of the Emperor's lover (she's a nobility but her house are full of debt from the Grand duke Heinrich) so their house were kinda annex and she can't escape from it coz she's afraid too for her young son. The emperor was also afraid that her beloved woman and son will get hurt so when the grand duke said that he wants to be a candidate for the throne though not a from royal family, the Emperor said yes.
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well, in fact he has a royal blood.. but they are not a direct descendants (because the temple didn't recognize them as direct descendants) so even though they have royal bloods, they have no right to inherit the throne..
(when he was introduced as someone who is also a candidate for the throne) they're referring to his "royal blood" but without the temples blessing and recognition he still couldn't be the king..
that's why he took the kings mistress and held her and her son hostages and threatened the king to acknowledge them as direct descendants if he doesn't want anything bad to happen to the hostages..
wait I forgot who was Heinrich??? ╥﹏╥